YANBU.
Just reading this thread has brought back really bad memories for me of a very dark time in my life as a newly single mum, living quite miserably on my own and having little adult company.
I was delighted to be going to a family wedding where I knew so many people and was so confident I'd have a lovely time catching up and having a laugh with siblings & their spouses, cousins etc. It cost a bundle as well in babysitting and hotel fees.
Going into dinner, I see the seating chart, and I was at a table where I didn't know anyone, so as I got to my marrying relatives mother and father in the receiving line, I asked if I could move to table where I knew everyone and there was a space as one person listed on the chart had cancelled at the last minute.
They said no, as the waiters had already taken away his space, and that it was only dinner. I resign myself to enjoying chat after the dinner, but...
I was sat at the table for all the old people - all between 70 and 95, where I only knew my great-aunt (I was 30). The highlights of the conversation as I recall was flower arranging and the weather of the various places they went on holiday. This wasn't the worst bit.
The sit down portion of the evening went on for 3 hours and I had the whole of that time to stew in my misery, wondering what I had done to be so punished by my own family members, and feeling the sympathy of my siblings who could see my situation. By the time people started moving, they all rushed up to the dance floor, and as I didn't have anyone to dance with, I was a complete wallflower.
My face was probably like absolute putty, so not surprisingly no one asked me to dance, and I ended up spending a couple of hours skulking around the fancy hotel grounds, and hiding in the bathroom, making an occasional appearance here and there so people wouldn't see me as needy.
I had a complete breakdown after that experience, and actually went to the GP for ADs the next week, as I was so tormented. (They didn't actually have any effect on my mood, so stopped soon after). This was a number of years ago, and whenever either of the marrying couple is mentioned, or I see them, I think about it, not deliberately but automatically.
I remember a couple of weeks before the wedding, the mother and sibling of my marrying relative was discussing the seating chart and that they were helping with it, and I asked laughingly if they could make sure I was with people I know. They assured me I would be.
The feelings I had/have irrationally included anger towards my whole family - surely one of them could have seen how awful it would be for me and just have rescued me?
Sorry for the long essay. I'm in a good place now and life has completely turned around, but there is always this awful residual feeling that will probably never leave me that the 'marrying relative's' immediate family didn't bother to consider me (even though I love seeing them and get on very well with them) just so some waiters didn't have to be inconvenienced by moving a seat around. I've never been able to talk to them about it and for my own sanity have had to forgive them, but I'll never forget the black cloud I was under.