OP I agree with everything you wrote. We are in the same boat. Dh's sister has sponged off my in-laws for about 20 years now. Married a dead-beat, moved into her parents' rental apt. with the understanding that they would pay rent, never paid a dime because they knew her parents would not throw them out. She divorced 13 years ago, no kids, and is still there living for free while my in-laws pay for taxes, repairs, insurance, heating etc. Why does she do this? Because she knows she can, her parents will never tell her to leave and she takes full advantage of that. She has a well-paying job and only herself to support. Dh and I have been married for 15 years, 2 kids, own our home and have never asked them for 1 dollar. Obviously there is a lot of friction between us and dh's sister. How could there not be? Parents of grown children who keep bailing them out of their mistakes do them great harm. If they do not treat all their kids fairly in terms of $ then they are only tearing their family apart. Grudges are inevitable. It sounds like your sister has no ability to distinguish right from wrong and so will continue milking your parents if she sees they are still willing to give her money. You and your other sister have every right to speak up and remind your parents that they have 3 children not one. Are they able to give you the same amount of money? Why not? If your sister can use extra money for luxuries I'm sure you can too. Who do you think your parents will turn to for help if they go bankrupt or cannot support themselves in the future? Your sister will be nowhere to be found. Speak up because at the end of the day every dollar she wheezles out of them most certainly affects the other siblings, be it them having to support the parents or,sad to say, in terms of inheritance.
Of course she will get mad at you because only in secrecy and silence can she get away with what she is doing.