What a tough situation to be in Miss B. In understand how hard separation/divorce can be. My parents divorced when I was around 25 and yes you would think it was easier but it wasn't. My Mum re partnered quite quickly to a man that I wasn't really sure of. We had a rocky patch for a little while but have worked things out. I was married around 2 years after the separation and my Mum's partner wasn't invited, not out of hatred but more respect for my father who still wasn't in a good place.
Now I certainly tolerate my Mum's now husband, and would never be rude to him but things are so much easier when she visits on her own. I guess it just feels like our family, we know our whole history etc.

I think your DF daughter is as you said one very spoilt and self centered person, and quite frankly I don't know how she can really be truely happy when she has so much hatred in her. So that being said I sort of echo what Tash said, I would just let your DF have his relationship with his daughter for now, who knows may in one, two, five years time she might reach maturity but in the mean time I really don't think she is someone that I would want to be around my child, I would worry about how she would act about a sibling that she had trouble even acknowleding. Focus on his children that are a part of your life, your son will develop his own relationship with them. It is her regret to have in life not yours.