paying off and unlocking an iphone

Alright, so here's the problem: My sister recently filed bankruptcy, and gifted an iphone 7 plus, that appears to have not been paid off(i assume because of the bankruptcy), to my mother(her step mom). So what I'm after here is to find out how much is owed(apparently my sister doesn't know herself), in the hopes of getting it paid off(it shouldn't be too much more given how long she had it) and possibly unlocked so that it can get used again. Obviously i'm not going to post the personal information beyond that here. hopefully someone from that can help find this information will get in touch in my DMs or something. I'm operating under the assumption it's probably a lost cause, but figured it couldn't hurt to try anyway(I lack better things to do). Anyways, let me know! thanks in advance!