*EDIT - The title is just clickbait. This is not about our dad having a new girlfriend, it is mainly about how he is acting and making everyone uncomfortable - Even with P's kids.

So this is a throw away, I know there isn't anyone in my family that has a reddit account but thought to create one just in case.
I (25M) and my Sister (27F) are stuck in a bit of a situation and even though everyone we speak to is on our side but I thought it would be good to get a neutral point of view in regards to our situation.

In 2023 my mum died a few weeks before Christmas and obviously my family were in a mess, especially our Dad (56M) more so than anyone else. I personally dont remember much during the first 6 months of dealing with my mums passing as I just blacked everything out. However with my sister she didnt get a chance to grieve properly as my dad kept calling her repeatedly to walk him through his own grief (which is not fair on her).

After our mum died, our Dad became very family focused and at first we were all there for each other helping one another walk through this obviously very tough situation, however over time (even after I came out of my blackout) it soon became apparent that myself and my sister were not able to grieve our mums death and we became a surrogate for his anger and loss. (We especially felt this after a situation in regards to the will which is another massive story in of itself.)

So cut to Jan of this year. We have our mums ashes, everything was normal-ish... or as normal as it could be after the death of our mum. We were starting to move on. My sister got engaged, myself and my partner are looking to move in together soon, our lives are looking up.

We had a conversation with our dad about the future and if he would ever feel ready to start dating again. He told us both that it is something him and our mum spoke about, but said he wasn't interested in dating for a long time. We both accepted what he said, but also reassured our dad that it was more than okay to do so, and our nan (mum side) gave him her blessing to do so as well.

This is when he rekindled his friendship with an old co-worker (who we will call in this story P). They worked together and have known each other for about 29 years and P has also met myself and my sister when we were babies. This rekindle ship happened after P's husband died... recently. (Only buried a few weeks ago)

Our Dad and P started to do days out together, nothing romantic it would mainly be to a garden centre or a pub of some kind, however from a once a week occurrence in January soon became an intense relationship. Our Dad tried to keep this hidden from myself and my Sister as long as possible that he was in a relationship, not only from us but from P's teenage children as well.

This is when it starts to become extremely messy. From our Dad trying to hide it from us and from P's children, soon became a very VERY public and out in the open relationship.

I live with my dad at the moment and with work wise I'm freelance, I was working at home one day and my dad sat down next to me and said point blank "how do you feel now that you have a younger sister". Which took me by complete surprise, as him and P at that point had only started seeing each other 2 weeks into the relationship. I don't know about anyone else here but I don't feel comfortable being labelled as an older sibling to literal children... STRANGERS that i have never met.

He then also stated to both myself and my sister that we should start calling P our Step-Parent. Which i'll be honest we had a mum and she's dead. P is a stranger to us. He may have known her for 29 Years, but myself and my sister have known her for 5 mins.

Thing is we don't care that he's seeing someone, we care about the fact that we feel ambushed and dis-respected that it's being so rushed to make it feel normal. It' is giving Brady Bunch Vibes and i'm sorry but it makes me want to flip off a roof.

Even our nan has said it is such a dis-respect not only to us but to the memory of her own daughter and to P's late husband with how they both are acting, especially with how P's children call us their older siblings and our dad their dad...

We don't feel as if we can say anything because whenever we've tried to say anything about our feelings with what's happening our dad has snapped and has been outwardly nasty to us, pretty much ending any and all topics of conversation about it. Will be honest though this has caused myself, my sister, her fiancé, my partner and our nan to get closer to each other... silver linings I guess.

Myself and my sister feel as though we're the after thought and that our dad has another opportunity with P's children. Especially with their family group chats (that we're not in), outings (which we're not invited to). And mainly feel like we're there whenever something goes wrong or to be a baby sitter service.

Whenever we are invited to certain places with P and our Dad, they behave inappropriately and make us feel extremely uncomfortable to be there. One example being when my sister and her Fiancé went to a pub with our Dad, P and one of her children. Our dad would have his hand high up on the thigh of P and her kid, the kid would be sitting on his lap hugging him, calling him dad (even though at that point in time our dad and P were together for 3 weeks) taking selfies together leaving my sister and her Fiancé to one side.

The thing that is even weirder is that whenever my nan shows up whenever P has been over, he has hidden P upstairs and done everything in his power to avoid her meeting my nan... even though they both have very publicly broadcasted their relationship online

It makes us feel so entrapped in a situation where we want nothing to do with it and being forced into something that we don't want to be forced into. As my nan said "it's not fair on the both of you, it's not fair on my daughters memory... it's disgusting how this is being handled".

Every mothers day since our mums passing our family usually likes to visit a Horse Chestnut tree at a park we used to frequent a lot growing up and is where we sprinkled some of my mums ashes. Sadly i was working and was not able to do this visit, however my sisters, fiancé went and laid flowers down to pay their respects. Only for them to see on Facebook that our dad had posted a picture of P saying "The most wonderful lady in my life" not even referencing his late wife... which he had done the years previously.

We both are just walking on eggshells now and we're both are just in need of some advice on what to do next... So AITA for being annoyed that my dad has a new girlfriend