Tools- I guess I'm the one that's crazy

SIL' s boy friend needs to be told his stuff is out in a week and be man enough not to whine to your SIL.
since part 2 ain't going to happen, explain to your wife that his stuff at your place is causing friction between her and her sister, and you don't think it's a good idea to keep storing his stuff as they're relationship is too important to you. :D

good luck :salute: