Also, the longer her family is there, they will have paid a larger and larger share of the equity in the property. When (if?) the house is eventually sold, who gets to keep the profit may become quite contentious. Given that the original sale of the house to the Beatles family was within the family, there may be a very complicated history that would make pocketing the $100k very difficult, and it is highly unlikely that the $300k house has $100k of equity in it that really belongs to the Beatles family.
Thinking about it more, the beatles family certainly hasn't paid $100k of equity in the few years they owned it, and prices in his area have certainly not risen 33%. They had to borrow $15k from his parents to "pay closing costs" initially. There's no way that $100k of equity is unambiguously beatles' to use. Some of it (maybe all of it?) has to belong to the in-laws, or to beatles' parents, but I'd guess very little to beatles and family. Which makes this worse. Beatles, you have a massive liability (taxes and mortgage in your name) with no access to the asset (physically or financially).
So, this is a very good time to initiate this conversation with the in-laws. You are in massive debt and it's clear that this situation can't continue indefinitely, which gives you an excellent reason to bring the topic up. It will be uncomfortable, but some plan must be made for how this will eventually unwind. Also, avoiding uncomfortable topics is what got you here in the first place, so time to change that behavior.