Did you tell him?

You need to tell him your feelings and concern.  Unless you reach a common understanding, the marriage will not work. You should ask him why he still needs to suppport his married brothers and sister?  You need to be confident that he will put his own family his top priority and he will respect you as his wife.  He can and should help others, but not just by himself after marriage.  It will be both.  He cannot force you to accept the way he wants to be. That's called respect. 

It's actually good that you raised your concern before marriage.  Now you need to be smart and analyze the answer from him, so that you truly understand what kind of person he is and accept him.  It should not be a surprise after marriage.