How can I solve this dilemma!? What should I do?

It's commendable you want to help your parents out financially. However, you do not have control over what your parents do with the money you send them. You are on the right track with realizing you should stop being so giving toward your sisters when they choose not to reciprocate. You can either send your parents the money and bite the bullet when your sisters get their "share" of the money or you can decide to not to send money to your parents.