I don't even think who sits where is what this is about. But I will say when it comes to physical closeness, proximity husband and wife have dibs on each other. That's natural and most people understand that.
Usually in seating arrangements, it is assumed that husband and wife will be together.
This particular upset is in my opinion a prime example of making a mountain out of a molehill. And I sense jealousy.
To take it to the extreme to alienate yourself from your brother? Then I don't think this is about how much you really care about him and want to be with him.
If any of my siblings of anyone else tries to come between a man and wife, that is me and my husband, that person will be the one who is alienated. A man and a wife have made a vow of union.
Better to accept that they are truly married.
And look at your own motives and feelings.
It would be much easier to get along.
Y'all shouldn't be fighting over him in the first place.
Something is really out of whack here.