The examples you give in your subsequent update are horrendous OP.
Taken together, it's clear that the pattern of exclusion & denigration you experienced while growing up have had a huge impact on your confidence, & left you in thrall to your mother.
This stood out to me though -
i.e. I can't say how she favours my brothers without dropping DSis in it etc
It's not true.
You can say anything you want to to your mother, without reference to your sister.
You could tell her the truth, you could list childhood examples, you could call her out in the moment ... but the fact is, even if you screwed up your courage & did so - she would not accept it. You've already said she'd kick off if challenged. So you are looking for a solution that does not exist, & the only way you will find 'closure' (spoiler - it doesn't really exist, in that "at last they've seen the light" movie-perfect way) is within yourself.
Your sister obviously challenges your mother, & maybe that works for her.
You have chosen - although I would be more inclined to say "been groomed into" a path of appeasement, hoping to forestall your mother's wrath, & probably hoping to finally win approval or affection from her.
But no matter how many nice birthdays or Mothers Day events you arrange for her - she will always favour the boys, take your for granted, & put you down.
So you need to have a long think about WHY you go out of your way for her, WHY you tiptoe on eggshells. WHY you fell you can never just blurt out "Oh FFS mother give your sexist old head a shake & stop fawning on the boys & putting the girls down".
This obviously goes deep with you, but for you it's 'normal' & so engrained that you possibly don't see your own role very clearly,. You have been cast as the Good Girl who will take mother's shit. The Patient Girl who can be unappreciated, but who will keep coming back for more. NONE OF THIS IS A CRITICISM OF YOU. You are stuck in a groove of tactics that kept you safe from major explosions or a refusal to even hear you as a child, & it's hard to see it when you are as enmeshed int he dynamic as you inevitably are. So I would recommend you find a counsellor who is experienced in toxic family dynamics, who can help you unravel your own feelings & responses, & come up with a healthier set of coping mechanisms.
For now - when you have some time, & can reward yourself with a small treat or distraction afterwards - put half an hour aside & imagine what it would feel like if you finally snapped & said something outrageous to your mother, like "fuck off with your favouritism you ungrateful bitch, I am done running around after you & paying for you to have nice events when you treated me to badly as a kid & can't even acknowledge how much I do for you now".
That probably isn't your style, but just sit with the thought, or something similar, & see what comes up.
Then take it to a counsellor.
Find one who is experienced in toxic family dynamics.
You have had decades of conditioning to accept second (3rd, 4th, 5th, 6th!) best, & it's going to take more than a forum post to sort out how to develop a strategy that best protects you from your mother's appalling attitude.
You don't owe her anything OP.
You have a perfect stranger's permission to drop the rope! So here's another mental exercise - what would happen if you didn't bother organising the next Social Thing for her, like Mothers Day or easter? How would that feel, what would transpire, what would you rather be doing instead?
Finally - stop believing that your DC need a relationship with the nasty old trout. You'd be denying them nothing but an undeserved sense of entitlement for the boy, (or, if he's emotionally attuned, dismay for his sister & cousin) & unfairness & similar feelings that you had to endure for your girl. Neither of them need that in their lives. I'm not suggesting NC - just slowly reduce the time they spend with grandma, & any influence she has over them.
outofthefog.website/toolbox-intro
Have a browse round this site - you'll recognise your DM in it
- a tremendous source of support & understanding. 