I'm going to pretend not knowing how to fix something for my in-laws so it doesn't become my responsibility next time
Why would you lie to your in-laws?
I just want to get rid of that problem. I think my in-laws should fix it. It is not my issue.
You shouldn't lie about not being able to fix something. Just set boundaries around what you're willing to do.
I'm getting tired of all the problems that place gives me. I'm going to ignore it and wait for it to explode.
You don't have to do things you don't want to, but it's immature to lie to your in-laws about it. You should have a mature conversation with them about it. Maybe you can suggest that they hire a contractor?