Need Advice On Estrangement

Hey all,

I'm trying to go no contact with my mom but not her side of the family and need advice.

Basically, I've never had a relationship with my mom, she left when I was 3 and has barely acknowledged my existence ever since. Growing up, I always had the fake presents on the coffee table, every school play missed, every major achievement or milestone skipped out on, and it was always on me to call her while she was playing perfect mum on Facebook.

A main highlight's from my 13th birthday. I got my first letter from her ever, and I was so excited I read it aloud to my dad and grandparents, only for the letter to be about getting a new baby sister and a potential step father who was closer to my age than hers (she was late 30s by this point). When I visited mom to meet this sister, turns out she was also 7-8 months pregnant with my brother. And then asked to borrow £50 for a takeaway that night.

I started low contact after another broken promise for my 21st birthday, she was going to take me to comic con as a way to "make up" for... well, everything. She even showed me the receipt of her buying 2 tickets as proof and she'd keep me updated for travel and accommodation costs, which I was happy to help with. Next thing I know she's showing me photos she took with Jason Mamoa and Matthew Lillard gushing about comic con that she went to with one of her friends.

We only talked during a group family call on Christmas and New Year's. Problem is, those phone calls always ruin the holiday for me because I've grown to have this bubble of anger towards my mom and I've had to swallow it for peace, swifly followed by anxiety from the anger bursting out and grief after the call seeing my siblings grow up through a 2 minute facetime session. This year, I finally bit the bullet and spoke to everyone on her side of the family but her for the holidays. I've got nothing against them as her parents helped me a LOT throughout life and are genuinely amazing grandparents, same with my aunt, and I can't fault my siblings for being born to her, I've even got hope that I can fully bond with that side of the family as there was no anxiety of having to deal with my mom, and my siblings are old enough for their own devices so I don't need to go through her now!

Problem is, now my Mom's messaged me on every platform asking why I've talked to them and not her. Is there any way to keep it no contact or do I have to swallow this anger every time I want to have a connection to my better relatives? Do I need to cut off that entire side if I want to maintain no contact? Is there a way of explaining why without things blowing up? Should I even explain it to her in the first place? My brain's been buzzing since she noticed after New Year's (yes, she didn't notice on Christmas that I didn't even text her like last year)

I tried explaining my feelings to her before but it led to me being ostracised from that side of the family for 3 years, and I really don't want that again!