Another wedding seating one

[Harriett1986]

Getting married this year. Top table is me, fiancée, both sets of parents and our siblings (I have a sister, he a brother). So immediate family only.

My sister has no partner, but his brother has a girlfriend. They’ve been together around 2 years (with one break up lasting a few months). They’re buying a house together.

I don’t want her on the top table because it’s immediate family only, and I don’t know if they’ll stay together. She’d be on the second most important table as it were, with the rest of his family (aunts, cousins etc) who she has met before.

AIBU? Ready to be told I’m cold hearted and mean...

Honestly just forget tradition and just sit people where they'll be happy and comfortable. I considered my guests when Wedding Planning and put them all with their significant others, of course they'll want to sit together.

This is not very relevant to the OPs question, but FWIW top tables aren't really "traditional". They're a relatively recent invention - part of the 20th cent wedding industry, along with "function rooms" etc. Until approx the mid 20th cent, most after-wedding parties were held at the bride's parents' home, and could be anything from a simple meal in a cottage to a grand evening ball in a stately home, depending on wealth etc.
This mid 19th cent pic shows a typical wedding reception at home:
en.wikipedia.org/wiki/Wedding_breakfast#/media/File:The_Wedding_Breakfast.JPG

Even Prince Charles and Lady Diana did not have a top table, as such:
natedsanders.com/princess_diana_and_prince_charles_wedding_breakfas-lot40195.aspx

For such a grand occasion, the small round tables are grouped quite informally, though of course, any table where the queen sits will be numbered "1".

Don’t have a top table. It’s dreadful.

Ugh hate top tables - so stupid - just let people sit where they want?

My then boyfriend (now husband) sat with me at the top table at my sister's wedding. We'd only been together for 6 months but were living together. He'd met some of my family, but sitting away from me at a wedding would have been his idea of hell.

My sister had no problem with this.

of course they'll want to sit together

Not necessarily reading the relationships board!

I was bridesmaid for my sister and sat on the top table. Top table was just bride, groom , both sets of parents , best man and me.

DH sat on a table close by with our DC , my brother , and some other close family members. The best man's partner sat with the other friends of the groom.

Everyone was happy and there were no dramas.

I’m baffled why this would be an issue as any one of the couples could split

Because if they don't, and go on to get married, at their reception OP is probably going to wind up on the kids table.

Just have both sets of parents on the top table, that’s kind of what we did.

Best Man was sat on a normal table, DH Brother was also sat on a normal table.

Good idea! Single sister can sit on her own. Perfect.