If you’re parents are fully aware of her nature…and are taking on the task of letting her stay with them…then on some level they might think that they can handle it.
Her being 30 or 40 or 50 has nothing to do with character. There could be plenty of people far younger than her…with toxic personalities. You will find respectful and disrespectful people in EVERY age group, Nadz.
When was the last time that you saw her? If it’s been a considerable time since you last saw her…perhaps she’s changed a bit. Who knows? Maybe in the past few months, things might have happened in her life (something as simple as self-reflection) to make her more grateful and even nicer toward others.
Here’s the issue. People USUALLY run into problems with others when they don’t establish boundaries in relationships. And in desi culture the whole fear of “log kya kahain gay”…or “family kya sochay gi” might prevent people from setting boundaries. But they’re important…and they need to be set early on. You don’t have to be mean about it…they can be expressed in a calm and pleasant manner.
For example, "Cousin…welcome to our home. It’s admirable for you to pursue your education and we wish you all the success. We hope that you will feel comfortable here. We want you to feel right at home like a member of the family rather than a guest because you ARE family. And every family everywhere has responsibilities. And our simple responsibilities include that everyone should have respect for each other, without which there can’t be a comfortable atmosphere. "
^There’s a calmer way to approach issues. And she can backbite you all she wants when she leaves your home. But hopefully her parents had enough sense to tell her to BEHAVE while she’s staying in your home.