Daughter vs Dads New Lady

What's the best way to handle when a daughter is jealous of her dads new girlfriend?
My boyfriends daughter has always liked me. I've been knowing him for 16 years before we ever started dating and his sister is my best friend so we all always hung out and my daughter would play with his daughters and nieces and nephews growing up. 
He got locked up for three years and while incarcerated that's when we decided to be together (no judgements petty drug offense, for Christ's sake the stuff legal now).
Now that he's out, his 9 year old is saying she doesn't like me but will never have a valid reason why because I'm amazing to her and always will be. When she is with me, she's doesn't act like she doesn't like me. In fact she is always all over me, her and her lil sister adore me and when we're together we genuinely have fun and do all the same things I do with my own daughter. 
He hasn't even told her were dating because he just got out and wants to wait for the right time. She's a smart girl and see's that my name is "My Girl" in his phone. Personally I think she already knows and is just mad because she thinks her dad is lying to her and maybe that's why she says she doesn't like me, and possibly because she is scared that he will leave again since he's been magically gone for three years. 
When the girls are with him I give him complete space to make up for lost time but hearing her say that he shouldn't have a girlfriend right now truly saddens me. To be honest since we're planning on moving in together I think it's silly to keep staying away completely when they are around. I think we need to start learning how to all be with each other. 
Any advice from anyone or anyone who has been in a situation like this before?