Daughter help

My SO has a son and a daughter, his daughter is 13 and son is 9. His daughter is very uncomfortable with my presence but I live with him so every time he has them over she's very uncomfortable, I don't want it to be this way and I can tell she doesn't like me very much (she peed on my toothbrush) and on Boxing Day she had a huge fight with her brother and dad and I believe it was provoked by my presence, I don't know how to make it less awkward when she's around or how to make her like me? I'm not motherly and I don't have kids myself, do any step parents have advice? 🤔🙄 I'll also add that his son on the other hand is great, shy but so lovely and never fails to be himself around me