Pestering filipino relatives?

The trouble with Philippine culture is that on the family level, we operate on the communist ideal. That is, from each according to his ability, to each according to his needs. That would be very hard to change. But try reminding your mom of the saying that "charity begins at home" and that her responsibilities are to her immediate family first, especially her children. But if she has something in excess of what she needs to adequately provide for your needs, I think you won't be able to convince her to abandon her sister. I'm sure if the situations were reversed, she might have called upon her sister to give her that same chance.