Immature mother and sister situation

My sister is 29, has a year old child with a guy who cheated on her (before that) and she knew. She still lives with the child and with our parents. He lives and works 5 hours from there in the country doing farm diligences. Yet she worships him but doesn't love him. She ocasionally travels to see him or he travels to see her but doesnt care much. He doesnt give her any money. Our mother cooks for her does the dishes, washes clothes and pays her career and gym all for her. Yet she's disrespectful, selfish, vain and intolerant.

The excuse for not living with the child's dad is she needs to finish her career.

When I take my daughter to her house everything bothers her and starts complaining. She also buys the same clothes I but to mine to hers or tries to show off when she is not the one having the birthday party.

My parents only try to help her by making her life easier but it only makes it worse, they don't understand either, they ignore my comments and continue supporting her. They treat me and my daughter very differently when she is not around and they know it.

So I decided to withdraw and see all of them only in very special occasions. So they can live in their mad house and because I dont tolerate such immature behavior.

However this damages my relationship with my parents and that's something that makes me sad and uncomfortable.

My sister and mother are attending to a shrink but this seems to have no effects.

Do you think I made the right decision? Do you see other alternatives? It's so difficult to explain the situation in words. I cant believe all this nonsense..