Your question "What is meant by the impossibility "A Buddha can not help someone not related to him, no relative of him"? may be explained by--understanding whether a person has faith.
When a person is able to (or willing to) believe in Buddhist teachings, then the conditions of that person have matured. There is a saying that "the Buddha is unable to help those who have no affinity with him".
What is affinity? It is the ability of a person being able to believe. Even a Buddha cannot help someone whose conditions have not yet matured. However, when they have matured, the person will have belief. Then the Buddha can help.
Here we speak of belief, is that first we believe in ourselves. This is where Buddhism differs from religion. In religion the most important criteria is to believe in God. In Buddhism the most important criteria is to believe in ourselves, not something outside of ourselves. Next we need to believe that we have the same Buddha nature. Believe that originally we were Buddhas. Believe that we are no different from the Buddhas. Believe that our true nature has become polluted and that once we remove this pollution we will uncover our true self-nature.
Once we have the belief, we then have to obtain the correct understanding of Buddhism. Buddhist teachings explained to all beings the truth of life and the secrets of the universe. Only after we have acquired a true understanding of it can we begin our practice. Therefore, practice is based on understanding. True practice is based on the foundation of principles and correct methods.
The ultimate goal of practice is to achieve attainment, to attain the real benefit of Enlightenment. The proper sequence of cultivation is 1) belief, 2) understanding, 3) practice, and 4) attainment.
Buddhist teachings in the Right Order of Learning is very important for us to first establish a correct understanding before starting our cultivation and practice.