Need A Perspective

Treat her the way you want to be treated. Then, if she disrespect, yell or behave abnormal, tell her in a calm cool neutral tone, 'be nice, you should not talk to me like this, if anything is bothering you I am ready to listen, but you have to talk in a nice way', repeat each time she raise her voice. You need to build a boundary to protect you. No need to tolerate bad behavior. It is always good to take a step back, introspect and look at the situation again with an open mind.

She may have her own reasons, but dont encourage complaints from your parents. If they can't handle her as parents its their failure, they cant expect you to fill in. Your sister may be feel cornered or controlled by all of you.

As she is going to marry, there may be stress, spend time in a good way, be her support system. Once she starts new life, she will also learn life lessons.

But, dont interefere in your parents and her relationship. You can build a good relationship with her. Adopt a neutral approach to maintain it. Give lot of space. Focus on your life.

 

Is it possible for any other extended family members to approach this subject with her ?
Maybe coming from some third party might help
Even with best intention when one sibling advices another one about how to treat parents it always end up in bigger fights.
Worst case if she is a bratty kid what can you . You cannot discipline an adult . Maybe distance will help. I’m sure the dynamics will change after marriage . Give it time .