Not another holiday blog! It's for my mum, I want you guys to tell me if I'm being a brat or not

My Mum's 50th Birthday is in February, so obviously its a special one. My Dad has already bought her a Mini Cooper- she's wanted one for forever as she learned to drive in one of the old-school ones, he found a good deal on a like-new one a few months back so he snapped it up for her.

My sister messaged me about going halves on a present, so we could get Mum something bigger, and I agreed... but now she's fixated on buying the two of them a spa weekend together. I obviously can't be involved because a) I can't afford a plane ticket to England right now and b) I will be busy with schoolwork anyway. But I don't really feel I should have to fork out half of a "Mother-daughter" trip that I won't even be going on? My sister completely ruined my last day with my Mum before I moved out here permanently. She was 24 at the time, and threw a massive strop because, you'll never guess.....

She had to sit in the back seat of the car. My whole last day ruined, because she had to sit in the back seat for a 30 minute car journey. My mother's back-seat at the time was nigh non-existent; it was one of those little Mitsubishis that claim to be a 4-seater, but can only seat people in the back if they are under 3-feet tall. Me being half a foot taller than my sister and having a bad back, got to sit in the front seat, due to the fact it would be even more uncomfortable for me to be back there. She was in a mood the entire trip, then refused to let me and mum eat lunch somewhere, because SHE wasn't hungry and SHE wanted to go home. And now she wants me to pay towards a nice weekend away for her and Mum. idk it doesn't sit well with me. We've all taken a step back from her since all that drama she caused when my SO was coming to visit for the first time. She's just too volatile to get close to anymore. I do think, if she's trying to build her relationship back up with Mum, it should be her paying for it, not me.

I wanted to get Mum something more permanent, and just from me and my SO. Dad has already bought the car so she won't have anything to open from him. My sister can afford to pay for at least a 1-day spa thing for the two of them, but it probably wouldn't be on her birthday. I want to get her some nice earrings, because she hasn't had any new jewelry for years now, and as it's her 50th my SO is willing to spend a bit extra to get a good-quality pair. I know she'd love the ones I've found too. I also went through our old photo albums and scanned all the pictures to my computer before I moved; I was going to fill a usb up and send that as well.

Buuut I don't know if saying "Sorry no I'm doing something else for her" now is going to cause another one of her tantrums. I'm always weary when I have more contact with her. My SO is always biased when it comes to my sister because she's been a humongous bitch to him too, so he'd never tell me if I'm the one being out of line for a change.... So we can be honest here! Am I being a brat in not wanted to contribute to it?