Please help. Step-family question: How do you make it easier on the children? My partner and I have been?

together for a year and a half now. He is separated from his wife over 4 years and they are still going through a messy divorce. His ex ran off with her tutor and when that relationship broke down she had a few others. My partner on the other hand was on his own for 2 and a half years and he said at the time he just existed. The problem we have is that the children (some grown up ones!) have all reacted differently to us getting together. I have a daughter aged 6 and she has after a 6 month spell being difficult totally accepted him. She misses him when he is away and writes him letters, hugs him and he is like a father figure to her. He has three children, a 21 year old daughter, a 19 year old daughter (who lives with him) and a 13 year old son. His middle daughter has been very welcoming and lovely to me, she and her partner even came on holiday with us. His son is a lovely wee boy and although I still sense it is difficult for him, he is always polite and seems happy when we spend time together. The 21 year old has I think never accepted that her Dad now has a partner (me!), as she stopped visiting him or coming to stay once we started going out, she also only calls him when she wants money and she seems to take the huff over times that he will not buy her something (and seems to think it means he does not love her) – latest being she wants a car for her birthday… my partner has pointed out that she cannot even drive, she already as a student cannot afford the £20 bus fares a month etc and that a car is very expensive including insurance etc but she was not at all happy. That said, I have only met her once and I often think how hard it must be for grown up children who have always known a certain family structure to suddenly have to accept a new person into their lives. I just wonder what we can do to help – so far we have tried to include her as much as possible but whenever we invite her anywhere she always says she is busy or does not want to go – the latest problem is christmas. We don’t live in the same city (not even close) and my partner decided that he wanted to spend Christmas with me (I offered to go to his). His son and middle daughter are ok with this as he explained that they would still have christmas day and that he still loved them etc His eldest however is just off the phone screaming at him about his priorities being in the wrong place and I feel awful. I wonder if we are being selfish. When he decided to come here it was partly because for the last four years apart from seeing his children for an hour in the a.m he spent a very lonely Christmas on his own, he tried explaining that to his eldest but she just would not listen. What should we do? Any outside advise would be most welcome. Not easy.