You probably know that I have 2 kids. I also have 3 neices. The older 2 are my kids ages and then there is the youngest who just turned 5. A few years ago we were having problems with all the kids because the youngest always ate less, got up from the table, etc. We (all 4 of us parents) enacted the "no one leaves until everyone is done" rule and it worked wonders. Kids are NEVER forced to eat everything on their plate but they encourage eachother to finish up and makes them feel like they are doing it as a team. It really helped my youngest neice sit and focus and helped the older kids to not play the "I'm full" card just to get up from the table.It's already gotten hard with the different families / different rules thing. At the dinner table for example, they're very strict that nephew has to eat what everyone else is eating, has to have a certain number of bites before he can eat dessert, etc. We're VERY lax about mealtimes; Sol pretty much eats what he wants whenever he's hungry (from a selection of mostly healthy options) so if he doesn't eat much at dinner we don't really care. I realize they probably think our approach is all wrong. Anyway, it's tough when they want their son to eat the grownup food and we're letting Sol have cheese and crackers or whatever.
We sit them at their own table and take turns being in charge. If it's Auntie's turn to be in charge of them getting up my kids know they have to listen to her and not to even bother asking me...although they like when Auntie is in charge as I'm more strict than she is. The 4 of us parents really all co-parent when we are together and it works for us.
I know you wouldn't parent that way but doing some things to make your nephew less stressed in those situations could help him. You could set him and Sol up at their own table and encourage them both to sit and eat...even if they are eating dif things.
Instead of focusing on what your sister and BIL are doing wrong (you probably wont change it) focus on what you can do to help your nephew while you're around.
High Priestess JessMess, follower of the Goddess of the Coiling Way and Confiscator of Concoctions in the Order of the Curly Crusaders