In some cultures it's normal to provide money family that can't provide for itself, like in "our" culture it's normal to pay taxes o the government can provide for those who can't provide for themselves. Because of this, I find it difficult to say your fiancé should or should not support his mother and his/her family. I think he should do what he feels is best, of course after he has taken your opinion into account, given that you're about to be married. (Taking into account your opinion doesn't necessarily mean he'll do exactly as you want - just that he considers your wishes)

Have you talked to him about this?