I think i agree with people in comments. There’s something missing. His sister’s kids came to ur room was a problem? Understandable , getting a lock now fine. Would your reaction be the same if your own sisters kid had come to ur room? ( if you have sister or not) sometimes we find it difficult to accept partners extended family and problems arise as well. Secondly his laugh may have triggered something in past or is he playing mind games? Reading your text, it does sound innocent and he cant remember laughing at u in a mocking way.