In my earlier post I was in no way saying that the parents were right to have such high expectations of their son that any 'failure' was intentional. I just wanted to challenge the previously stated assertion that children are not able to do this at two and a half even after being used to using nappies.
How many potties do they have in their house? Could you buy them one as a gift so they have more than one? Could you suggest that your nephew might want some privacy to use the potty so they shouldn't be around him when he does it? Perhaps this would allow him to relax a little and not be sitting there waiting to get hit my one of them.
Is your sister disgusted by the mess? Does she hate having to touch the wet clothes and sort him out?
She needs some hints on how to deal with accidents but I haven't really read anything other than 'don't make a fuss' has anyone got anything more step-by-step this-is-what-you-do? Could you pass on some old clothes to them to use for a while or tell them that you have a particular cloth and bucket which you keep for cleaning up and that your dc put their wet clothes straight in the washing machine? If everything was a bit wet I used to stand mine in the bath and swoosh the shower over their legs to freshen them up. Maybe there is too much room for her to use freezing cold water there
but it would be a hands off way to clean him up.
Since reading your first post I keep seeing your little nephew in my mind's eye and he is flinching away from a big dark figure.
I hope he can be things get better soon so that he can avoid this awfulness but please don't feel bad about yourself if you can't change your sister's behaviour towards him. It is not your fault.