I would think that due to your upbringing that you should not want to contribute to the same problem for your sister and BIL. My MIL was a hoarder and never did I give her stuff I didn't want. In fact she would offer me stuff and I would take it knowing it was going right to the thrift store. Also it seems to me that they have been kind to you in many ways like living with them. It does not have to be your problem-you can choose to be kind and thoughtful.