Soon to be step dad - Help!

Hey everyone, so I've been seeing this amazing girl for a little over a year now, and things are getting serious. I'm moving in with her in less than a month and she has a 5 year old daughter. I've never really had that much experience with children, but I'm not opposed to them either.Majority of the time everything is perfect for our soon to be family, and I've had solo play dates with the little one while mum is at work, and we always have a good time, but recently I've been feeling some resentment from her. She will tell her mum that "she doesn't want people having sleep overs anymore" and when I ask her to do simple tasks (reinforcing discipline with mum) she gets the shits with me, or will ignore me.She doesn't have contact with her father at this current time, (he's an abusive drug addict) but she keeps glorifying him, even tho he used to hit her and all that terrible shit. I don't want to replace her dad, but at that age I feel like she needs a stable father figure in her life. I just don't want to come across like an invader to her life, or that my discipline principles will make her hate me.I know that communication is key with my partner, and she agrees that she's been a bit soft on the discipline side of things too. But I'm just a little lost with the situation and would appreciate any tips or advice! via /r/Parenting http://ift.tt/2tqE2go