My fiances daughter

Okay so how do I deal with the fact that my fiance already has a daughter? I mean we don't see her bc of his baby momma.. But we've been trying for a baby since the beginning of October and nothing... I mentioned something about why have a baby with someone you knew you wouldn't stay with and his response was... I thought I was gonna stay with her... She messaged me today asking me to tell his sister to take down the pictures of their daughter bc she didn't give her permission to take those pictures and I just started talking to her to get her to help with the child support situation... Idk what to do right now... Does it bother you guys when you talk to your significant other and they talk to you about how they were with someone and they thought they were the one?? Like how do I do this??? The ones of the little girl are his daughter.