Step daughters 18th meal

You are not her “non biological” parent, you’re not her parent, you’re her father’s partner.

Your father was abusive and your step mum let him be abusive. When did society change? When we woke up that women and children were people with feelings and children would act out their feelings. It stopped being all about what men wanted. We started to recognise that children didn't just 'adapt', that they could be emotionally harmed by divorce and it is unreasonable to just expect them to accept a step parent. We also expected men to be able to parent and not hand everything over to a Step Mum. The OP and her DH should have addressed and talked about what the SDD was feeling and the OP back off. But then very often it means the DH has to actually do the parenting. No child would be hit because they aren't shutting up and putting up.
The OP and the Mum's partner isn't wanted there by the people who have the right to go out to celebrate a milestone birthday.

No its her parents putting her 1st on her 18th birthday as they should not you or your wants.
Honestly op I'm beginning to understand why she doesn't like you

Yep, kids should just stfu and pretend they feel something they don't in order to make it convenient for the adults who had agency and made all the choices for them. Certainly they can't be allowed to show any distress at the situation!

I'm sympathetic with OP in this situation too, as it sounds exploitative, but that's coming from DH.

Yes a very odd turn of phrase.

It’s the SD you don’t get along with? Then why on earth would you be going to this meal?

So who is that you don't get on with? If its the stepdaughter then ofcourse you wont be wanted or invited

Why do you think your ‘right’ to be there trumps her wishes for how she celebrates a milestone birthday? No wonder she doesn’t like you if you’ve just centred yourself for the entire nine years you’ve known her.

An invite would have been nice but the mother is trying to plan something her daughter would like so your feelings aren't paramount here. She hasn't invited her partner either so it's not even like she's just excluding you.

A meal with just her parents sounds like something the girl has grown up not having much of so this will probably be lovely for her.

Me and my ex have taken our kids for 18th birthday meals. I can’t stand him on a day to day basis but we have a nice time at these type of events, not pretending. He didn’t make the comment, the mother did.

The obvious answer is you and your dh also take her somewhere. Why would you want to be there with his ex if you don’t even get on?

Op this is down to your dp it was his role to do all of those things not yours. I expect his daughter knows it. But is taking it out on you. Let them go for the meal. I think it is nice for children with separated parents to occasionally do things together because they chose to have her.

It wouldn't be nice for the person who matters most - SD on her special day - as she will be aware of tensions between the adults.
Arrange another meal/extra celebration.

I voted YANBU until I read your update about not getting along with SD. Why do you want to go if you don't get along with her? Let them crack on without you.

The reason you feel like a free nanny is because you have been a free nanny for 9 years.

I have two step sons (both my partners had a son each from previous relationships).

It's a minefield and often works best if the adult is prepared to sometimes accept that they are a bit put upon.
No one doubts you've put in the hard miles, but I think it's reasonable for an 18 year old not to want the step mother, who she doesn't get on with, to be absent from her celebration.
Just run with it. It's only a meal out.

OP my parents divorced when I was10 and my father remarried though my mother didn't. From time to time I can remember spending time with my parents, who were on good terms, but without my stepmother. My stepmother didn't like it very much when it happened and created tension about it which I picked up on and which felt oppressive, especially when I got dragged into it - for example, I remember my father coming to chop some wood for my mother, for which he took off his watch & rolled his sleeves up. Afterwards he forgot his watch when he left, so my mother told me to give it to him next time I saw him. I did so in front of my stepmother, referring to the wood-chopping afternoon, and the atmosphere was immediately tense - she hadn't known he'd been over to do that and was clearly displeased.

These sorts of incidents made my childhood uneasy & miserable. I did not dislike my stepmother but, rather like your stepdaughter, felt a certain betrayal of my mother if I enjoyed myself with her. It was a confusing & unhappy time.

She died in 2021 by which time I had known her for 50 years and had come to love her. She had many very good qualities.

It can be hard for stepchildren

So you don’t get on with her and expect to go to a dinner to celebrate the birthday? YABVU.

It would be weirder to invite you when stepdaughter doesn't get on with you. Possibly they have been considerate by also not inviting step dad (mums partner) to keep it even.

Doesn't sound very healthy

I personally don't think it's odd at all. Ultimately, they are her parents, and it's healthy for them to have a stronger bond.

It's great that the parents can put their differences aside to be here for her for an important milestone.

I would invite someone who didn’t like my daughter to her birthday meal either.

It’s perfectly normal for an 18 year old to go out with her DPs for her birthday. We took DS out. It’s a celebration about her reaching adulthood. It’s not about showing how inclusive she can be and how super all the step parents get on. It’s her meal, about her. I think it’s nice that her DPs can get on well enough to have a meal just the three of them.

Because she’s jealous of her husband and his ex wife spending any time together, even for their daughter’s 18th.

Why do you even want to attend, OP?