You can't stop everything that happens, but you can certainly enforce your family's rules in your own home. All you have to say is that, "IN THIS HOUSE, we don't say things like that or we have to sit in a chair for a minute" and use the timer on the stove, with the girl in a chair in sight of it. The next infraction ? 2 minutes.
She may hate you, but it's YOUR house, and you have the right to enforce YOUR rules in YOUR house. If your sister in law doesn't like it, then she can teach her child to behave in an appropriate manner.
I don't think kids HAVE to share their toys. But by the same token, your child doesn't have to share the toys she is playing with, and that's okay. They should be allowed to make some decisions over the things they own. If you are at "the brat's" house, and she won't share, simply say, "You don't have to share that one, but are there some other toys that it would be okay for your daughter to use ?" And then, if your daughter is having more fun with the toys she is playing with, and the owner tries to "steal them back", you can jump in and say, "But you told her she could play with these. Do you want to share the other toy now ?" Give her some choices, and some control, and maybe that will help.
The other option is avoidance, which doesn't help family relationships. It's a tough spot. I don't envy you !! But in the end, when your daughter grows up to be caring, respectful and polite, and the other is causing major trauma, you will no longer look like the "mean" parent. You will look like the intelligent one. :-)