NSH. I think the friend jumped in because she was friendly with your sister and thought you were wrong to not let her stay with you, but now she wants to reframe it because in her eagerness to be a better person than she thought you were being she didn’t consider that you know your sister and her children better than she does and you might have insight she wasn’t privileged to know.

It’s madness she made arrangements behind your back and now wants to hold you accountable for a decision they made? Yeah...no. I’d tell her I absolutely will not be accountable for my sister’s responsibility, I will pass along messages and encourage her to pay you but that is pretty much all I’m willing to do.

Good luck to the friend getting the cash though. Sounds like she’ll need it too because the sister either through unfortunate circumstances or irresponsibility didn’t have resources for emergency accommodations (?) so she may not have it for a broken computer either.