My mother is a christian woman but has strong traditional roots in voodooism and ancestral worship.My question is,Is it possible for human beings to invoke a deva/gods/territorial spirits(nagas etc) to do our bidding? In a Buddhist context Can a Deva help a human being? Is there such a story in the suttas.Why would a Deva help a human being? According to my mother a particular deva helps us because he was our family/relative/ancestor.Is this compatible with Buddhist Beliefs?
In Buddhist scriptures, devas are described as powerful beings who often interact with humans. So assuming devas are real, they can surely hinder or help humans in a material way. What no being can ever do is force or hinder another's spiritual path. There we are truly alone. Even Mara can only tempt beings to stray from the skillful path but can never succeed as long as we can see through its illusions.
Yes they can. If one does good, particular "higher" beings (including Buddhas) can more easily help that being (they are attracted to the good karma). If one does evil, asuras can more easily haunt that being (they are attracted to the evil karma).
There are many other properties to them that are completely irrelevant to the Buddhist project (see Noble Truths) and the importance of meditation and attainment within our extremely fortuitous but also extremely brief life-time...
But to answer your last question... yes it is compatible but also remember that Hindus also believe in this stuff. The difference with Buddhism from all (inferior) traditions is there exists Buddhahood for the sake of all beings and the Three Trainings and this has little to do with over-concern for worldly matters (e.g. Mythbusters and other obsessions with paranormal phenomena). The Buddha himself said this and so did Confucius (who had Enlightenment).
To satisfy your curiousity though, devas are mentioned throughout the scriptures and the Buddha mentions his experiences with them in vivid detail.
The following is a taste of the first half of "The Mind Experiment" with excerpts from respective suttas:
The “devas” (collective name for heavenly beings) are not above men or outside the world, not transcendent but immanent, worldly beings. They all have distinct features and occupy certain positions in a vast hierarchy of levels, each heaven providing specific qualities of life and environment, body and consciousness. There is a Lord God in most realms but no one is in charge of the whole; there is no super lord, no unique and supreme ruler of the universe, no divine creator of all and everything.
Some Gods believe themselves to be eternal and almighty because of the exceptional qualities of their existence – such as an extremely long lifespan, unwavering happiness, extensive presence, power, and knowledge – which makes it difficult for them to “see things as they are” (impermanent, suffering, non-self, empty; Chapter 1). They enjoy such bliss and brilliance that it is difficult for them to realize that all beings are, in essence, equal; that their godly state is just a matter of natural and temporal retribution, the result of their own excellent karma, and that they too are subject to the universal law of causality, to suffering and frustration when their karma runs out. They experience decay, death and rebirth (in a lower plane). By themselves they do not discern ultimate truth, but when they did upon hearing the Buddha’s teaching, “they beat their breasts and wept” (AN ii 33).
Gods are finite because they exist. They are delimited and conditioned by their own thusness, by the very attributes and properties (skandhas) that make them divine. They are “imprisoned in their person” (SN iii 85). Although none of them are responsible for the world or for other beings, they do have more-than-human powers. Their destiny highly exceeds our human one, because they have been much better than we have been; it is their excellent karma that took them there. Therefore we should emulate them for the moral and spiritual qualities that earned them such splendid existence. As religions know, they can appear on earth and interact with humans, they can protect us and intervene on our behalf; they are affected by our prayers, which they can answer insofar as  their power extends to us, and  we have the spiritual capacity and karmic affinity to receive their help. But there is not much they can do to alter man’s destiny or their own.
Gods and heavens are part of and evolve within the whole of existence, or they would not exist. They are no exception to the general rule but subject to the universal process of origination-cessation just like the rest of us, ruled by cause and effect. They evolve according to the natural law and order of the universe (Dharma). Heavens and hells, gods and angels, spirits and demons are not a matter of mythology but of cosmology.
There is much more to reality than what our senses and brain register. From the human level, the pure worlds above are perceived when the mind ascends in pure concentration (dhyana-samadhi). “sursum corda” – lift up your hearts – is to be taken literally. To “contemplate God in Heaven” has been the main purpose of all religious exercise: to cultivate a supernatural awareness through prayer, moral excellence and mental purity, single-minded faith and contemplation. That means essentially to develop dhyana-concentration, to shut out the world of the senses, to silence the distraction of thought, to rise above this earthly-bodily dimension and “unite with Heaven.”
At times when spiritual cultivation was more developed, gods and heavens were a natural part of life, as can be seen in the traditions of Greco-Roman culture, Judeo-Christianity, Islam, Hinduism, Daoism, and other religions. All of these offer observations that match one or more aspects of the heavenly reality as it has been elucidated in Buddhism. The Buddha’s omniscience covered the whole of reality, and his revelations about gods and heavens have been, and can always be, corroborated by people with adequate samadhi power. There is nothing exclusively Buddhist about it; any spiritual adept can discern the superstructure of the universe.
The Buddha once described a heavenly voyage undertaken by one of his followers, to whom “the thought occurred: “I wonder where the four great elements—earth, water, fire, and air—cease without remainder” (in other words, “what is beyond the physical universe”). And that brother attained to such a state of mental concentration that the way to the deva-realms appeared before him.” After having gone through the consecutive realms of the lower heavens and having put his question to the gods there (who referred him each time to a higher instance), “he became so absorbed in concentration that the way to the Brahma-world appeared before his mind thus pacified. He went to the gods of Brahma’s Assembly . . . then the Great Brahma himself appeared . . . and finally, as swiftly as a strong man flexes his arm, he vanished from the Brahma world and appeared in my presence” (DN i 215-222).
The Buddha himself proceeded likewise. Once asked about the whereabouts of a group of his followers who had recently died, “he sat down, thinking over, considering and concentrating his whole mind on the question of them, making a resolve: I will find their future, their fate after this life, whatever it is. And then he perceived the destiny of each one of them. In the evening, after he emerged from meditation, looking bright and shining . . . [the Buddha said:] ‘Then an invisible spirit made himself known . . . and appeared himself before me, a splendid presence’.” It was the spirit of a former king who explained his rebirth among gods as a king of non-humans, who now wanted to engage on the path of enlightenment and was asking the Buddha for advice. (DN ii 204)
On several occasions the Buddha described in detail the rebirths of disciples who were recently deceased, also mentioning their actual status on the path of spiritual realization. He advised his students to see this for themselves and explained a manner of self-inspection, like looking in a mirror, to discern clearly one’s future and make sure that: “I have destroyed hell, animal-rebirth, ghost-realm, evil fates, and sorry states. I am a Stream-enterer (first stage of the path), incapable of falling back into states of woe, certain of attaining Nirvana.” (DN ii 92-94) When he spoke about rebirth, he frequently warned against being reborn in the “descents” and “states of woe,” among animals, afflicted spirits and miserable ghosts, or worst of all, in one of the hells.