I feel so bad for my mum but at the same time I really don't.
Yeah, you really shouldn't.
She is creating a monster out of your sister, if she hasn't already. (Surprise us - did your sister ever offer to share her spoils, or speak up that maybe the huge sum of money for her new house should be fairly split into 2 hefty deposits instead of 1 outright purchase?)
I'm sure you already have a handle on some of the dysfunction, like the dynamics of Golden Child vs: Scapegoat or Neglected Child, so I won't bang on about that. Besides, it's not useful in terms of changing any of the unfairness, which is obviously & very reasonably eating away at you.
My mum tells me she helps me all time but I feel like she delusional
Very probably, but again - so what? No matter what her psychology & motivations are, the effect on you is the same - you are being treated unfairly, & that has a host of other issues buried inside it, such as your own feelings about being valued, heard, appreciated & loved ...
So my suggestion, dear OP, is that you begin a gradual process of teaching yourself to give less of a fuck. I know that sounds glib, but as you cannot change your mother, you can only change your response to her.
What form that might take can only come from you, but if you look at statements like:
I had 3 jobs at university and paid all of my bills from age 21. At 22 I paid full London rent
i had to have my own back
even if I need money, I simply deal with it
whilst not asking/or expecting a penny
I have recently taken a pay reduction to do a job I have always wanted
& compare them with:
At university she never had a job as my parents supported her, from age 21 - 26 she paid no rent
[mum] flat out bought [her] a brand new house ... kitted out with luxurious new furniture
she complains how she is so hard done by, even though she got bought everything
she didn't save any money (but had a great time buying designer clothes, extravagant holidays and plastic surgery)
& ask yourself - which of these scenarios paints a shallow, spoiled twat, & which one shows a woman with backbone, a committed & focused work ethic, & genuine self-reliance?
I know that doesn't change anything concrete, or alter your mother's disgraceful favouritism.
But it could be the beginning of a different way of handling your relationship with your family.
You might also want to find the "Stately Homes" thread/section here (am sure PP can signpost you if needed, apologies I have forgotten how to). It's a forum for adult children of dysfunctional parents, & is jam-packed with empathy & advice. You will find many links to reading resources there too.
It would also help for you to engage with some form of counselling, to help you assess how you want to tackle making changes to how you respond to your mother's batshittery in future.
TL:DR
YANBU!!!
You are the better person.
Get some external support to ensure you truly believe that, & pat yourself on the back for your resilience & independence while you're doing it :)
