DF was 30 when he had a 2 year affair with a 20 year old and DM kicked him out. I was 8. He went on to marry OW and they are still together almost 40 years later. DF began bringing her in the car to collect me for the weekend almost immediately. He told me she was his friend and she was lovely to me. DM took the split very badly (understandably) and instantly told me the truth and that he loved OW more than "us" (not her). She would refuse to use OWs name and called her "that slag" until the past 10 years-ish when she now sneers her name out. DM bad mouths them every chance she gets, even today. DM remarried when I was 11 to a (lovely) wealthy gentleman and so would sneer at DF having less than her and everything she had being better. This also allowed her to progress her career so she liked everything about her being "better" than DF & DSM. She has never been in a room with DF alone since the split. Handovers would be done with us being left on the driveway when he pulled up - no conversations. She stopped him seeing us when we were little and he took her to court. It took years and she made things very very difficult for everyone involved. We were not allowed to speak his or OWs name when we returned from weekends. She would ask how our weekend was and, if we fell into the trap of answering about something we did, she would say "oh aren't they wonderful, how wonderful of them to take you to the cinema. How amazing they are" etc. We quickly learnt to say "ok" or "boring, didn't do anything" and hope the conversation went away quickly. This pleased her. We were not allowed to attend his wedding when I was 10. They went on to have 3DC (who I adore) but DM refused to call them our half siblings and showed no interest in them. Oddly now we are all adults with children of our own and she comes into contact with them at my house she is lovely to them and acts happy families. But ignored them for 25 years and belittled everything they did. DM would remind me weekly that he chose his new family over us and how happy they must be. Every time they did anything or went on holiday she would ask me why I hadn't been invited and pointed out every difference. If he ever asked to move or skip a weekend she would delight in telling me that they were doing something family orientated and I wasn't invited. She also told everyone we know what he and OW did so I always felt awkward talking about him or saying anything positive about DSM.
On the other hand DSM (OW) has never said a word about my mum. Her and DF would politely ask how my mum was and, if I had visited family etc, how my aunts/uncles/grandparents were. They showed an interest in everything I did and said it was lovely and how lucky I was etc. They did not attend any nativities or events of mine growing up, as they knew DM would demand they not come and cause a scene and tell everyone around that "that's my ex and the slag" and make it harder for me. Instead they would ask for videos or ask me all about it when they saw me. They did not come to my wedding for the same reason. I informed DM & DF that I was inviting everyone and please could they get along. They both said yes but DM began with her bitchy comments almost immediately to me. Also began pressuring me to put them near the back and have DSF on top table etc. She reminded me of every single (amazing) thing DSF had done for me and how he was my father. I did not mention any of this to DF but he clearly knew and informed me he was not coming as it was my special day and they would celebrate with me later. They would not take no for an answer. He later told me they watched me walk into the church from his car. My half siblings took lots of videos and photos for him from the day. As a teen if I spent summer holidays with DF, DSM would quietly enquire when I last spoke to my DM & request I ring her "for a catch up" once a week. My DSF also never bad mouthed them. The worst he would do is sigh and raise his eyebrows in agreement when my DM dragged him into arguments to "tell her how awful they are". He has come into contact with them over the years (if DM sent him to do drop offs if she knew they were around or whatever) and he is pleasant.
As an adult I can see what DF & DSM did was extremely wrong. They both knew what they were doing. They have discussed it with me and both understand the way they went about it was wrong. I fully understand DM being devastated. However I cannot understand the lengths she went to and how, even now, she feels she did nothing wrong by involving us. I have friends who have been in worse situations with cheating ex's and not involved the children. I also do not understand how, after being happily married herself for 30-something years, she still hates them. On paper she got the better end of the deal. A happy, loving healthy marriage to a wonderful man and her life made easier by money. However she is still very bitter.
I find it hard to hold a grudge against DF as their split meant the introduction of 2 wonderful humans (DSM & DSF) who I love very much and my half siblings/nieces/nephews etc. Also the opportunities opened to me by DSFs money. I feel very sad that his behaviour has led my DM to a lifetime of anger. But it didn't have to be that way. DMs behaviour has made our relationship extremely strained and I struggle to have conversations with her and my life is very much "boxed". She is kept away from a lot of family gatherings and conversations due to her behaviour. If she is at my house I feel panicky incase DF pops round unannounced. I feel none of this with DSF as I know he would be polite. I do a lot of BBQs and parties with my half siblings as we have same age children and I, obviously, cannot invite her and then struggle to tell her about it. I still have the "it was ok. Boring really" mentality with her.
When we bought out first home DM said she would gift us the deposit. They had done this for their other children/step children. Her one proviso was that DF was not allowed in the house ever. I declined. DSF then secretly gave it to me. DM doesn't know.