AIBU to refuse to attend my nephew's birthday when my sister excluded my partner, then expected me to apologise?

Your sister is a professional victim. We have one in our family and it will always be the same. To not invite your partner or your/her step dad to a 1st birthday party is pathetic. Just ignore her. I wouldn’t bother making any further effort with her. I’m sure she’ll come crawling back around Christmas when she wants some gifts for her kid! That’s normally how it goes.

I disagree with this. If her sister is like this all the time, sometimes you have to put your foot down and refuse to give her the apology she wants. I have someone like this in my family and it becomes exhausting dealing with the constant drama caused by them, and then they always expect an apology afterwards even when you’ve done nothing wrong. Then the cycle just continues because they think that the person apologising is doing it because they are at fault, instead of realising you are just trying to keep the peace.

You do know its OK to decline invites and that's perfectly normal don't you?

Yes, I do know that. Just shouldn’t do it because you can’t go anywhere without your partner!

You can decline for any reason whatsoever...including not wanting to go without your partner.

Of course you can. The other party is entitled to think that the reason for declining is ridiculous too, though.
Anyway she didn’t decline because she can’t go without her partner. She declined because she had other plans. She just had to chuck in the bit about her partner not being invited to make a point.

Oh for heaven’s sake , it’s a 1 year old’s birthday party! So much fuss , we just ha a couple of friends from our anti- natal group at home and a bit of cake. Ridiculous drama.

You do know you can attend things without your partner and that’s perfectly normal don’t you?

Some details changed privacy reasons

I just need some perspective here please.

So, a few months ago, my sister texted me and told me that she was holding a 1st birthday party in May for my nephew, Reeson, and that she would love for me to be there, but that my partner and our stepdad weren't invited. For context, our stepdad has been in our lives for 15 years...
She said that she decided to make it a joint function with Reeson's nan (my brother-in-law's mother) at the last minute, and that there was "no room" for too many other guests. Mind you, she and BIL invited SEVERAL of their friends, but the only people on her side of the family that were invited to Reeson's birthday party was my parents and I.
I am not familiar with R's grandmother at all, and we have only ever made polite small talk if we've ever been at a function together. My BIL in law and I also do not have the greatest relationship, though I believed it was starting to improve until this happened.
Now, it also happened to be my anniversary with my DP in May anyway, so we had made plans well in advance of the invite. I politely and respectfully advised her that, whilst we had already made plans for our anniversary, I wouldn't feel comfortable attending the function without my partner in any case, but that I was grateful for the invite, and that I hoped they had a lovely day.
He is my life partner and as much a part of Reeson's life as I am, whether she sees it that way or not, so I would assume that she would have understood my position, however, clearly she didn't because she simply responded "Lol". I replied and asked her to clarify what she meant, but she did not.

As a result, we then didn't talk for several weeks. To be clear, she did not make any effort to contact me, to ask how my family and I are getting on, nothing whatsoever, just radio silence.
A mutual friend then intervened, and told us both that we were being childish, and that we just needed to talk and clear the air of any misunderstanding. I took the feedback on the chin and subsequently texted my sister on 3 separate occasions and I told her that I loved her and Reeson. I invited her round for a cuppa and a chat and to see if we could sort it out, but I did not receive any kind of reply for several weeks.
She then contacted me out of the blue in August and said that she believes we could rectify the situation, but that she was hoping I "had something to say". In retrospect, it's easy to see now that she was waiting for me to profusely apologise for refusing to attend without DP...
I replied and told her that I didn't have much to say as I had already had time to process the event and move on from it, but that I do believe our wires had been crossed and that there was a communication breakdown. Again, I invited her and Reeson to my home for a chat because I was off work, but it seemed that my response was still not satisfactory enough, because she simply responded that she wasn't prepared to move past the issue, and that was that.
I have since texted my brother-in-law to ask how they're all getting on. He received the message but did not respond.
My sister and I have not talked since.

I'm really hurt that she has made no effort to contact me whatsoever. She hasn't once asked about how myself and my family are getting on because she clearly doesn't care, and I'm not sure if I do anymore.
The irony of it all is that she hasn't asked me ONCE how my son is doing either 😂

I dont understand why some people issue invitations but exclude people/partners. It was a 1st birthday party for a baby. Hardly the event of the century. Your sister is being ridiculous. Youve tried, you dont owe anyone an apology. Rise above it.

You do know you can attend things without your partner and that’s perfectly normal don’t you?

You do know it was their anniversary.

You do know it was their anniversary.

You do know you can attend things without your partner and that’s perfectly normal don’t you?

I dont understand why some people issue invitations but exclude people/partners. It was a 1st birthday party for a baby. Hardly the event of the century. Your sister is being ridiculous. Youve tried, you dont owe anyone an apology. Rise above it.