We used to take turns with mil doing Sunday lunch, she lived round corner but an hour on a Sunday was the only time dd ever saw her as mil was looking after six grandchildren between her other two DC.
Whenever it was her turn it would be just her, but it ended up being whenever it was turn she'd turn up with at least three of the kids, feeding them wasn't a problem as they didn't eat -any- much food anyway but mil didn't supervise them, they would go up and trash DDs things emptying era toy out and dd hated it, I stopped them going upstairs which semi pissed mil off as dd should "share" and mil likes to relax on a sunday!
Muddled along for about a year when she turned up with the kids as I was dishing up and told me her DS and his gf and her child are coming and she'd told them I'd make them lunch, I ended up taking off mine and dhs plates to put on theirs but it was doable, no sign of bil an hour later so we sat to ate when mils phone went off, it was her other DC saying they'd be home soon (they'd had weekend away) and if dinner was on the go to which mil said the more the merrier or I couldn't stretch another two plates out so when they turned up they took Dhs and mine up home and left the DC who'd been with mil all weekend.
I was seething but too fucking chicken to say anything, after all the cleaning up had been done and we sat to settle in front of to three hours later bil rocks up with gf and dsd, they'd called at a mates and Lost track of time, gf cheerily asks if she can do anything to help and looked a bit put out when I replied "I can turn the dial on the microwave just fine". If she'd complained I'd have flipped, if she wanted freshly cooked veg they should have turned up, they knew what time lunch was.
We knocked it on the head then,I told mil I don't mind if I have some notice, but pouncing it on me when I'm dishing up and then a second time when I'm about to eat is taking the piss, that even Jesus would struggle turning food for three into food for 12 :)
A party though, while I wouldn't take extra uninvited guests I'd not be too bothered if my sister brought a mate or something. There would be plenty of nibbles but as long as they bring their own booze (our family always do this) and weren't known to be a cunt in drink I dont think I'd mind.