Children with other people! Kinda long but please read!!!

Jeslynn • My name is Jeslynn,currently TTC baby #2 wit my fiance. We've been together going on 3years <3 i have one beautiful, amazing daughter that i wouldnt trade for the world! Love being a part of glow!!
My fiance and I both have a child by previous relationships. His daughter is almost 7. He and the mother has not been together since she was 1. He and I have been together for over 2years now, live together, and plan to be married in September. My daughter adores him but he has been in her life since she was 5months old. But his daughter shuts me out! I knew it would be harder for her cause she's older, but it's been 2years and he has her 4days a week! Sometimes she'll play with me, and do things with me, but most times she just sits and won't even speak to me and even when I try talking to her! And she's like this with everyone, including her own daddy most times! She will not have anything to do with my family, nor anyone in his accept his mother. She won't even hug my 3year old! Yesterday, Easter Sunday, my fiance had to meet her mother at 330, they have to split holidays, and she was talking to my mother, which shocked everyone cause she never talks to anyone, and said that she wish we could all have holidays together, which is understandable, but that her mommy didn't want to be around me, and didn't want her around me either. And I have been never said anything to this woman, never even talked to her, cause her grandparents normally keeps the child when it's her time, and me nor my fiance has to deal with her. But by her saying that, I know her mother has been saying stuff about me. She even said her mommy said I was a mean person!!! And I HAVE NEVER SPOKEN TO THIS WOMAN!! I'm wondering if this is why her antisocial attitude comes with us, and of course her mom says horrible things about my fiance.. But even my fiance will say that there is something wrong with the child!! She won't speak, won't play, won't hug anyone, won't do anything!! She's only 6!! Normal 6year olds do not act this way!!! Should I just give up on ever trying to have a relationship with this child??? Idk what else to do!!! This might sound mean, but I find myself dreading when she comes to stay because the tension in the house changes! And the even my fiance don't wanna talk because she goes back and tells her mom everything that's said and done at our house. It's soo hard!!! Please anyone, advice????