Quite a backstory here.
Just after DS1 was born my dad offered for DS and I to move in whilst my DP studied and worked abroad (he came back regularly but for short periods of time) so we could save up/help him out with DF's mortgage payment etc. This was a 2 year arrangement.
5 months in, my dad found a new girlfriend - she's not much older than me (irrelevant really) but she never ever liked me. She was very spiteful and my dad would intervene but I felt like I was holding him back and we agreed I would only stay for another 7 months.
9 months in, my dad told me that him and his DP had decided to move in together and 'she needs' me to move out as soon as possible. I agreed but was very hurt obviously.
We ended up moving 250 miles away as we could not afford locally (London) and both DP and I could find more work up here. This caused a lot of resentment from my mum towards my dad, as she felt he allowed his girlfriend to drive us out.
Nearly 4 years on, they are now expecting a baby together - I am in regular contact with my Dad and he adores his grandsons but I never, ever see his DP. I reached out to her via text wishing them congratulations. I also sent another a few weeks after saying that I never really understood why we didn't see eye to eye, but I hoped that could improve especially as I would soon have another little sister.
She however told my sibling (who is only 16 and she does get on with) that really I didn't have any relation to 'her' baby as my dad wasn't biologically my dad (he adopted me)
I am hurt by this as I have never viewed my dad or other siblings as anything but family. I think I should probably take a step back for my own sanity (had a pretty rough few months without this situation) but at the same time I love all my younger full, step, half, whatever you want to call them siblings and I love my dad and do not want to lose contact with him, but how on earth is that possible with his DP so clearly adamant that I am not part of any of it?