Wow I just read this and I am so sorry for what’s been happening OP.
I have a sister like your mum except she switches which sibling she prefers at any one time. She is a half sister (from my dad) and my mum always indulges her as her mum died when she was 7, our father died and so did our brother (I was the youngest when bro and dad died) my sister is nearly 20 years older than me.
She comes across as meaning well for all she does but it’s always tainted - I got married 2 years ago and she asked to do my hair knowing full well my best friend is a hairdresser (my sister works in a hospital) I politely declined as obviously my best friend would do but asked her to do the invites as she is very crafty and loves making cards etc.
Anyway long story short all hell broke lose- I was ungrateful and care about my friends more than family etc. Suddenly my other sister was best thing since sliced bread even tho she hadn’t spoken to her for months due to taking a dislike to my DN.
I actually ended up seeing a councillor myself and you know what it was the best thing I have ever done. I would highly recommend it - I do still have a relationship with this sister but it’s on more equal terms and I take no crap. My mum sometimes says need to give her special treatment but I am strong that I am not pandering anymore we either have an equal relationship (as in her and me not my siblings) or it won’t work.
When I was getting married I was guilt ridden and at one point didn’t think she would come but was first time I had been strong not to back down - my councillor asked “what’s made you be strong about this” I said “it’s my wedding, this is it the one day in my life I can have things as I choose” my councillor said “that’s really sad that you feel you can only have one day, you spend so much energy appeasing everyone else that you are only allowed one day for you?” This struck a massive cord with me.
I don’t think you need your mum to see a councillor she is not going to change, I think you do - go and vent and explain all to a third impartial party and get their perspective it honestly changed my life I am mid thirties now and happiest ever been. I still have a difficult sister who flip flops between favourite siblings, she offered my DSis money for a house deposit but not me but I honestly don’t care. It does help that me and my siblings all accept she is difficult and they have been supportive of my change of attitude.
I have another friend who is full NC with both her parents due to favouritism of her sisters kids compared to hers - she is much happier that way but her parents don’t seem to care- I would say your mum does seem to care she acts and does things terribly but there is caring there.
Obviously it’s up to you I really wish you good luck xxx