Aitah for leaving my family home bc my brother (m35) had my under wear

my brother(m35) recently moved back bc of legal/drug problems and it has been a living he'll since then. but the last straw was when I (m30) doing laundry and I notice that my brother had a pair of my underwear from when I was in h-s in his things to wash. I also want to add they are x-s and barely fit me in high-school so definitely never have or will fit my brother. Right then I brought it up to my mother bc this is really concerning that he has kept a pair of my under wear for about 15 years. and her response was I thought those were you exs and that I should be worried bc they are not mine, she spent that whole day trying to convince me that he hasn't been keeping my underwear for some reason. They tried to convince me I was insane then rather then just admitting he stole and has kept a pair of my underwear for 15 years. It all ended with my mother (f62) t3lling me boys will be boys and refusing to acknowledge what just happened. Am I the a hol3 for leavening. They even lied to my extended family and they won't even talk to me now. So I have what can only be described as a horrible brother and a mother who is going to let him do what ever he wants even encourages him to harass me at this point