What I did was buy a couple of extra potties from the op shop and have one in each room.
They also had a child's seat which fitted over the toilet for poos, although they could use the potty if they wanted.
I don't think you'll change your daughter; she has a hard job rearing two little ones with developmental difficulties.
I wonder if she has regular professional advisers who might get through to her that she is doing far more harm than good?
Otherwise, all you can do is point out to your son (who must have seen this angry behaviour) that these wee boys have enough to cope with, without an angry mother making them worse.
If you stay with them, just tell them what good boys they are; praise them and encourage them in every achievment. Be a haven for them and reassure there is nothing bad or dirty about toileting.
Every living thing gets rid of waste one way or another. Maybe you can turn that into a game?
Hippo poo and giraffe wee-wee; footballers, singers, pilots and puffins - all do it
Heated Brush recommendation,please.
NEVER EVER HAVE I - Game 2 (Alphabetical)
Angela Rayner lashes out and calls Sunak “pint sized loser”.