My sister-in-law asked my mother for a black check and I want to ask her why.
What's your reasoning for asking your sister-in-law about the check?
The stupid bitch wanted to spend $377 on a fucking table lamp.
I can understand why it would make you upset, but it's rude to ask people about their personal finances. You shouldn't worry about what other people spend money on. If it was fine for your mom to give her money, then it shouldn't be your business.
It's not fucking fine.
Ultimately it's none of your business and the finances of your sister-in-law and your mother are not for you to be concerned about. I agree that the expense was extravagant, but your opinion on the issue isn't relevant in this situation.