Please help. my partners 12 yo daughter is very demanding and wants everything to revolve around her.?

Im a 33 yo male with full custody of my 2 young boys ages 3 and 6. i met the most amazing and strong willed women 8 months ago. she has a 12 yo girl, 10 yo boy and a 5 yo girl. my partners husband past away 2 years ago and they have obviously had alot to deal with. there was a lot of simalarities that brought us together.the problem with the 12 yo is she want everything to revolve around her. where we go, what we do and where we stay. we both have our own houses and if me and my partner want to spend nights together it always has to be at her house. the week nights are taken up with the 12 yo over comited activities( 4 nights a week) and even on sundays for at least three quarters of the day aswell. we have the other 4 children to think about also. and when we do things for them or take them places, she becomes this little bi*ch. the other main issue is myself and her mother, connot show any affection what so ever in front of her because she really goes off. that even includes a kiss good bye, a cuddle or even holding hands. now i really do understand that her father died and now she sees another man with her mum, but its like she wants her mum to be miserable for ever. my partner is so happy, she always tells me i made her smile and laugh again, and now thats all at risk. my partner wont stand up to her cause shes afraid of the back lash from her daughter and now my partner has even cut out week night stays at her house. i hope it all makes sence. thanks.