DP and I have been together for almost a year. Our relationship is serious and we plan a future together. He has two DC aged 22 and 19 from a previous relationship. DP and the mother of his DC had been separated (he moved out of the family home), for over two years before I met him. I had nothing to do with the breakdown of their relationship or break up of their family unit, but his ExP is intent on treating me as if I am the OW and making me out to be the enemy to their DC.
The back story is that they were together for nearly 30 years (met as teenagers), but grew apart and wanted different things from life. For the last 10 years of their relationship, they co-parented under the same roof but led separate lives and slept in separate bedrooms. He stayed because he couldn't afford to maintain two homes and wanted to see his DC as much as possible (he worked long hours). She was always a SAHP who did not work. When he left, he gave her 2 years notice to live in the family home with the DC (whilst he continued to pay all mortgage, bills, car and living expenses), before selling up and splitting the proceeds 50:50 when his youngest turned 18. He then loaned ExP £50,000 of his share so she could afford to stay and buy a flat in the same area. She also got first pick of all their shared belongings from the family home and he gave her the car, but he no longer pays her bills, living expenses etc. He supports his DC by giving them a regular sum of money directly (his DS is at college, DD going to Uni in September). In my opinion, DP has been very fair to his ExP and is a devoted, responsible father who dearly loves his DC and wants to do right by them.
I have never met DP's ExP or DD. I have met his DS, (who is disabled) a few times, as 'daddy's friend' and he has accepted me without issue. DP wanted to introduce me to his DD recently, but she told him she isn't ready to meet me yet. Fair enough. We both respect her wishes and DP has told her we will take it at the pace she is comfortable with. DP tries to see his DC as much as possible but this is irregular and his ExP always makes it difficult to arrange.
This weekend, DP's family were having a little get-together at his parents' to celebrate 2 birthdays and for us to see them before we go away travelling. DP had been trying to arrange seeing his DC before leaving for weeks and ExP kept brushing him off with excuses, then at the last minute (Friday), ExP says he can collect his DS that night and she will drive his DD down on Sunday, because she (ExP) would like to have a catch up with DP's family herself. DP explained that they were more than welcome to come, but to be aware that I was there too. ExP got very angry and laid on the guilt-trip about putting his DD's feelings first and making ExP feel ousted from her 'own family' (DP's family). She then issued the 'her or us' ultimatum.
So DP was stuck, obviously wanting to see his DD but not wanting to exclude me (FWIW I get on well with all his family and they like me, I was looking forward to seeing them). We decided together to relent this time so that I would make myself scarce for the duration of their visit. It meant I missed out, but DP got some precious time with his DD. I'm not sure we did the right thing. I worry it has now set a precedent and ExP will expect to have her demands met every single time.
I appreciate that from ExP's POV there is a lot of change to process and a lot of emotion to deal with in a relatively short space of time (downsizing, empty nest, having to get a job and manage finances, bills, expenses etc and being independent for the first time in her life), and I am not unsympathetic to her, but I think she is behaving very unfairly.
I have never been a step-parent and don't have any DC myself. Although I have many years experience as a Nanny, this is with much younger children so when it comes to teenagers I'm clueless. Of course I don't want to force a friendship with me on them, but I am a big part of their Dad's life now and I would just like everyone to be amicable and get along, particularly for DP's sake as he's stuck in the middle 
Any wise MNetter's tips or advice for things I could do to facilitate harmony in this situation would be greatly appreciated.