A number of Suttas make very clear the practitioner's duty to his/her parents. Does the Pali Canon say anything about duty to siblings?
Yes in several places Pali Canon says to help one's relatives such as parents. In the Mangala Sutta in one stanza says to support mother, father and cherish wife and children while in another stanza says to help the relatives. So relatives should include siblings, aunts, uncles, grandparents etc.
"To support mother and father, to cherish wife and children, and to be engaged in peaceful occupation — this is the greatest blessing"
"To be generous in giving, to be righteous in conduct, to help one's relatives, and to be blameless in action — this is the greatest blessing"
Although I am unable provide any other exact instances, there can be other instances too.