My mother allowed me to cut contact with my father when he met my SM and this lasted for many months.
When they moved.... My DM was glad as it meant she didn’t have to see my Dad anymore and I didn’t feel I was allowed to be sad about it.
I never had Christmas there until i was an adult because I didn’t know how to ask my DM.
This isn't about your SM at all. This is about you trying desperately to not upset your mother. Your mother has always been bitter about the breakup, and as a result she made you feel bitter about it. She stopped you seeing your dad as soon as he met Jenny. That immediately set Jenny up as the problem in your young mind.
You haven't done things you would have liked to do in order to not upset your mum.
You don't want your mothers grandchild looking to Jenny as anything other than 'Jenny' so as not to upset your mum
Your mum has always been the adult in this situation, and you have always been the child. Your mum, as the adult, should have put her own hurt aside to allow you to have a better relationship with your dad. She didn't, at now at 30 you are still tying yourself in knots, upsetting other people, just so your mum isn't upset.
It's not good enough. I completely understand that you don't want to upset your mum. But upsetting your SM, your dad and your sisters instead isn't the way to go.
And I bet your mum still won't be happy regardless.