HELP!! My boyfriends little brother is stealing my panties.

So I (20F) have been living with my boyfriend(20M) and his family for about 9 months. On Monday morning (its friday) when his mother and I were cleaning out the bathroom we found 2 stuffed animals that had been cut open and sodomized and a hoarde of undergarments while cleaning them out we notices 2 pairs of my underwear cover in fecal matter and what looked like s*emen we also found a pair of panties belonging to my friend covered in the same stains as well as multiple used condoms. NOBODY and I mean NOBODY in this house owns stuffed animals other than him and everyone else has a partner. Now that being said when it was brought to him by my boyfriend, my boyfriend and I's friend/roomate K and his mother he denied it heavily. Ive asked that he be kept away from me. No contact. While I understand it may be hard due to us living under the same roof I dont understand why he still feels the right to talk to me, be near me or go into mine and my boyfriends room. When ive brought these situations up to his mother she indirectly implies im overreacting and that she understands he needs help she cant afford it. All im asking is for her to keep her son in check before I lose my shit and report it to the police. Can anyone help me navigate this situation?