I don't want to pay back my sister-in-law the $4000 that she gave us.
Why don't you want to pay her back? Are you mad at her?
Yes. I'm angry at her because she is so rich but acts as if she is poor.
Even if you don't like the way your sister-in-law acts, not paying back the money you owe is rude and selfish. It doesn't matter how much money she has, you still should pay your debts. If you're unable to pay it right now, talk to her and tell her when you can expect to be able to pay her back.
What I do with my money is my business. I didn't ask for the money. She just gave it to us.
Oh, you didn't ask to borrow money and this $4000 was just a gift? If that's the case, it's very odd and rather rude of her to suddenly ask you to pay her back. However, if she's asking for money, maybe she's financially struggling and needs the help right now.