AITH? Went to baby shower my family wasn’t invited to.

Okay so a little backstory. My sister was with her ex from the time I was 16 to 33 (Currently 38) it was a toxic relationship most likely due to them having kids at 17. They have 4. (Currently 21, 20, 16 and 14). My ex BIL had cheated on my sister which is why they divorced. I have always been in my Nieces and nephew’s life and have always been there for them since they were babies and even more so during the divorce which was nasty. Both of them are currently remarried, ex BIL to the mistress. My sister had always tried to persuade the kids not to have a relationship with their father although my nephew wound up moving in with him. When I found out what he did to my sister I went off on him. Besides that we have always had a mutual respectable relationship, he was like a brother to me. I may have spoken to him since then 5 times when he would ask me to talk to one of the kids about something or ask my opinion on something but always having to do with the kids. Fast forward to this past summer after my sister married her current husband who is younger and just not responsible (heavy drinker and drug user) and she went off on me saying she was dead to me bc I have talked to him in the past. Back to topic my 16yr old niece got pregnant and my sister kicked her house so she started living with her older sister and brother. My oldest niece was working 12 hour days and supporting her and setting up her house to help when the baby came. Well I had days off at work due to my boss being away so I had scheduled a trip down to Florida to see them. A month after I planned that my nieces told me that their father was going to host a baby shower for her and they wanted me to be there so they made it during my trip. I told them of course bc I have never missed anything of theirs even though we don’t live in the same state. So a week before I was to leave I was at my mom’s house and she went off on me for going to the shower since she wasn’t invited. Mind you I didn’t know at the time she wasn’t invited but niece explained that during the gender reveal she had stated that if her daughter couldn’t go she couldn’t so my niece said why invite her if she was going to say no. Also they said they were only inviting those that were not trying to push an abortion and those that wouldn’t cause drama. So anyways my mom and sister said I betrayed the family and that I was disloyal. So am I the asshole for going to my nieces shower just bc my ex BIL was hosting? Mind you my sister wasn’t planning a shower of her own and told my niece that if she was invited she would go and be nice to her ex but no one believed that would be the case.