Quoting Chuck Bass:" It gets complicated because the neighbors are FIL's cousins, and the house we live in used to belong ... [snip!] ... powerwheels moved from where we left it (he doesn't like where we leave it because it's easily seen from the road I guess)."
ummm... i don't think i'd give a smurf if he was family or not and be changing the locks. i'm sorry, but that's just creepy. if one of my family members are going to be at my house when i'm not there (sometimes my brother comes to hang out in between his college classes because it's closer than him going back home), i at least get a call/text asking if it's ok and letting me know they'll be there. i'm sorry if he's used to meeting with his friends over there, but that's pretty damn inconsiderate of him. i really would change the locks.