Some backstory: Growing up my mum and I didn't have a fantastic relationship. I think diagnosing with today's terms she is a bit of a narcissist. We argued a lot - I was a fairly standard emotional teen, and I didn't feel she ever prioritised me. I have a sister 18 months younger than me and I always felt she favoured her, although my sister today does complain mum was a crap parent. As kids my sister complains she was in my shadow as I was outgoing, she was shy.
In our university years (our parents really wanted us to go - we are first generation university) my sister and I started at the same time (I took a gap year) and the adage "it's always the quiet ones" is true as my sister ended up dropping out of uni secretly, stashing our parents' cash, went to Ibiza with a MUCH older man (older than our parents), then having a baby with him. She's 18 today! My sister eventually ended up moving back in with my parents when that went south, then went on to a local university. She met another guy, ran away and got married and fell pregnant again before the end of this degree, although she did stumble through and complete this time. He was the same age as her but followed a very strict orthodox religion, which she converted to for a while, today neither of them practise.
My sister didn't get a job after university - the religion her and her husband followed is very traditional. She had more children - she went through some horrible miscarriages too, she did not have it easy. However, her husband is a bit of a child and has never worked for anyone else and is always jumping from one business of his own to the next and they are routinely in serious financial difficulty. Always being bailed out by his parents or mine. My sister has had to return to work recently (in a trade) and is doing really well for herself - it doesn't support a whole family though.
My mum and my sister have a very close relationship as adults. My sister and her three kids go there every week as her current job allows. I did when I was on maternity leave and a career break for a couple of years but I now work in a very intense job and I can't. When were the three of us I can't stand the dynamic - we've regularly gone on holidays all together and I just hate myself when I'm with them, and how it makes me feel. My sister reverts back to a child and baits me into competitions. Both my mum and sister have disordered eating and they competitively undereat. I am overweight but more mentally happy with my appearance but I feel horrendous when I am with them. As an example a couple of weeks ago I went to my daughters dancing show and invited my mum and sister and afterwards my mum laughed cruelly at an overweight teenager in one of the dances "no one told her dancing really isn't for her", it was truly awful, but my sister and her enable each other she would never have said her thoughts out loud if it was just her and I. My mum feels awful for my sisters situation (financial) and is worried for her all the time. On the other hand, she makes comments about my job (I am in a very senior role and earn well) "why do you work so hard" ... "why do you have a nanny"... etc, she worries so much about my sister's lack of security but can't be happy for mine. I try and talk to her about things and she is so disinterested she just cannot engage, so if I confide any work stress with her she tells me I am "ridiculous" for staying in the job, she'd just walk out. My mum has been quite stressed recently due to complications with the sale of my deceased nan's flat and I have listened to every concern she has over it.
Fast forward to today and my AIBU...
I booked for my mum and I to go for a fancy afternoon tea to celebrate Mother's Day (a week later). I didn't invite my sister for two reasons - a) it's not something she could afford right now and it would make her feel crap she couldn't come and b) it would be nice to have some 1:1 time with my mum. My mum recently when I followed up asking if all still good for this Sunday said "is <sister> coming? I tried to persuade her" I said I hadn't asked her as didn't think she could afford and she said "dad and I gave her some money for <some family task> and told her she had to spend it on treating herself." Then this morning my sister text saying "Mum keeps asking me to come on Sunday, is there room to add me on".
I'm so upset my mum can't imagine just her and I doing something and now it has to be the pity party for my sister's bad life choices all day! And I will probably be picking up the bill, too. Her and my sister do things the two of them frequently / I've always felt a bit smarted every time I hear they've gone Xmas shopping for a day, or a garden Center or something. They've never once invited me. So why is my mum begging my sister to come on a day out that I had arranged with her?