Trigger warning ⚠️ SA

My SO ( 46 ) and I (24) have been in a relationship for a year now. everything is great. I love his son, his family, everything is great except that I cannot stand his SIL or her youngest daughter. she is not his biological niece but to him that’s his niece and I cannot stand her. Every single time she comes she wants to be on top of him, jumping on him and if we are laying in bed she will come in the room and lay in between us. That bothers me completely. He will completely let me go to hold her or whatever. Now idk if it’s because I was sexually abused as a child or maybe my daddy issues. Anyways so he told me that if I continue to have an issue with her they we will break up. He claims I’m jealous of a 4 year old and I say it’s trauma Thoughts ?