Ok, so your H can’t rock the boat - for whatever reason - but fundamentally agrees that this isn’t right.
he’s only saying that the supervision is enough because- the n his position- he has to.
so YOU have to tell him that you’re going to take the hit/flak on this and that you’re going to say that after the intentional drowning attempt and the repeated attempts to get your kids out of your view so they can be hurt again - without consequence- that the kids won’t be attending any event for the foreseeable future.
in the meantime love, it really might be worth getting them some kind of self defence training
fundamentally this is a failure by the kids parents to contain this situation. It does sound extreme and that there is a REAL danger to your kids. I understand that no parent wants to admit that their child has severe issues, and that’s the journey that they must make without your suggestion, but your kids are not going to be hurt again.
my DS is now 20, I was (still am) Mumma Bear as far as he’s concerned, but he’s 6’2” and a 90kg boxing monster now, if I had been in this situation with him and any other kid, I’d have gone ballistic.
my own mother hurt him deliberately once, that was the last time he went to her house.
my dad was mean to him (as he was to me my whole life)
haven’t seen him in years either.
we’ve got one job. To protect our kids. You have that role and you have to do it.
all the hand wringing in the world with “oh he must have some condition or whatever” is irrelevant
what if nephew kills one or other of the kids? Then what? What good is a fucking diagnosis.
sorry, I’ve wittered on, tell H how it is and that you will assume all responsibility for the fallout. All he has to do is stand behind you and back you up.