Sisters always looking for a handout

How much influence should my boyfriend's family have over weather or not I should be going through with the long term relationship? My Boyfriend of many years is a great guy and we are planning on getting married. The problem is his family. His parents and younger sisters seem to take advantage of him ( at least from my point of view). My boyfriend has a well paying job for a single guy, (when we get married I would still have to work to contribute to the bills) and has no children to support. His sisters do have children and are always wining the blues, mostly about their financial situation. His parents seem to think that because the sisters have children and he doesn't, that he should help them out. He sees it like that too. He always says he does it for the children's sake. He doesn't want his niece and nephew to go with out. Both sisters are married and have jobs of their own. I don't see why they are always looking to my fiance to support their families. When we go out for family meals, he always pays the bill and the tip. Every family function that we are involved in he ends up paying all their ways. They do nothing for him in return. I do not want to enter into this marriage feeling that I owe his sisters something because they have children and I do not.