hello OP. I am your age but male.
You've got a range of opinions in the thread. My guess is, having once been 18m myself (although I've never done this!), the kid is _NOT_ into his aunt. My guess is he wanted a quiet place to jerk off, and the smell and general ambience of a woman *may* have compounded it.
I do not think this acceptable and he absolutely shouldn't be doing it in your room... but humiliating him for it, might only make both of your lives worse. If it is as I suspect, this kid will be horrified enough and wish he was dead already.
(If it is the 0.01% outlier where he's a serial killer or wants to fuck his aunt or whatever... OK... but chances are...)... kid will already be ashamed enough. Say nothing, put a lock on your door if you think he's a repeat offender/pantie sniffer/whatever. He probably won't be. He's probably just a horny teenager.