Ugh, MIL's basement and garage are full of mostly worthless stuff. We've tried convincing her to go through things and declutter, as she may need to move to a senior-friendly place at some point, but we realized over the holidays that this is simply not going to happen, and that we will have to deal with it when she is gone. DH is an only child so it will indeed be us, even though we live 600+ miles away. There are a few articles of furniture that I would like, but that's about it.
Thankfully, my parents have moved numerous times and downsized each time. They have had to deal with the belongings of their own parents and grandparents in the past, and are not leaving so much stuff for us to deal with.