How should I react at how his ex wife treats me?

My bf has been married before, he has a daughter that is 4. Well I have watched his daughter over the summer and completely fell in love with her. We get her this december but his ex wife found out that Jake (bf) is dating me and doesnt like that. She feels as if I am trying to replace her as a mother. I love Ariel and Jake but I am so confused. I dont want to take her place as a mother but I know I am going to stay in her life so what should I do?