How can I (29F) support my brother (36M) who has a horrible child (10M) that's ruining his life?

Here's everyone involved:

Me, 29F

My husband, 29M

Our daughter is 5 months old

My brother 36M

His wife 34F

Their kids, 10M in the title, and 6F

A few months ago my brother and his family came to visit us. It had been a very busy and exhausting few weeks since I'd had my daughter like anyone with a newborn will know. I was feeling well enough to see them and let them properly meet her.

We were in the backyard eating lunch. My niece found a furry brown caterpillar in the bushes. The kids were told not to touch it because it could be poisonous. Around 20-30 mins later, my daughter starts screaming. Right as I looked and started going over to her I see my nephew stepping away from her bounce seat. I instantly knew he'd done something. There were red blotches all over her neck and cheek. When I picked her up, the caterpillar was there on the seat where her head had been.

In total panic my husband and I rushed to the ER. My brother and his family stayed at the house. By the time we got there the whole side of her face had swelled up and she was shrieking nonstop. Doctor diagnosed pretty quickly, there were caterpillar spines lodged under her skin like we'd assumed.

My nephew must have broken off the twig with the caterpillar (he hadn't been stung) then as soon as no one was looking while my daughter was asleep, he rubbed it on her face.

I called my brother from the hospital, he confronted my nephew who eventually claimed he just "accidentally dropped it". He had no remorse as usual.

My husband and I had to be interviewed by a child welfare worker. She talked to my brother who drove over to explain what had happened. It was a very long ordeal, I didn't blame her or the hospital people for doing their job at all and in the end my husband and I weren't at fault for doing anything wrong, but it was very exhausting to deal with.

This was the absolute last straw for us. I was furious with this child. Enraged doesn't even begin to describe it. I was/am also frankly terrified for my brother's family even though I know how stupid that must sound.

That weekend my husband and I had a long conversation with my brother and his wife on videochat after they'd gotten home. They are well aware something is very wrong with their son. They don't downplay his behavior at all. He scares them. When they punish him, he doesn't care. He doesn't react to any criticism, yelling, or logical argument, but he'll find a way to get revenge even if it's weeks later. He also doesn't care about rewards for good behavior. There's very little he openly cares about that they can use as leverage, and he seems to outright prefer doing destructive things more than anything they might take away or try to bribe him with.

He's arranged tacks sitting upright on the floor outside their door that my brother's wife stepped on.

He's pooped on their bed and once left poop and urine in every drawer of their clothes.

He floods the bathroom by letting the tub overflow and leaves the gas stove on. He'll use the stove to play with fire when no one's around, and several times has set off the smoke detector in the middle of the night.

They were all getting sick except him at one point and my SIL caught him mixing cleaner fluid into some leftover soup.

He has done countless things to hurt and scare his younger sister like cutting off her hair while sleeping, putting chili pepper powder in her underwear, telling her someone is coming to kidnap her or kill their parents, and once caught a garden snake that he dropped in the tub while she was having a bubble bath. Those are just the ones that first jump to mind.

He also is cruel to animals. Pulls wings off insects, cuts up worms, thinks it's funny to scare cats and dogs. They had a couple hamsters, one mysteriously died and the other they thought had escaped until it was found in a box under his bed with no legs.

He tries to blame things he does on his sister including the caterpillar but my brother knows what's going on. Doesn't stop him from constantly lying to anyone and everyone.

They do not know what to do. He's apparently very well-behaved at school, so his teachers have never seen any of this. There have been a couple times when he injured himself on purpose and then told a teacher he was being hit at home. Resulted in a CPS investigation both times. It was also conveniently right before parent-teacher conference week. My brother and his wife were never able to bring up their concerns or have a good rapport with those teachers then because they clearly believed they were child abusers.

They've brought him to several child psychologists but he lies to them, either acting perfectly normal or telling them the same stories about how his parents are crazy or abusive, so they can't take him to anyone now in case they get investigated again. They're terrified that one day an accusation is going to stick and they'll lose their daughter, not to mention the legal consequences.

I told them that as long as their son is hurting people, lying, and incapable of behaving, he cannot be in my daughter's presence. They have abided by this since then.

The problem is, we miss seeing each other! They live a couple hours away and we used to visit a few times a year. They're already socially isolated because trying to manage their son takes so much of their time and energy. My SIL's family has also banned him from being around their children. He's been banned from other kids' houses and between that and the rumors that my brother and SIL are abusive, they've been slowly frozen out of their school community. They have a few childless friends still, but like I said they hardly have any ability to see them. They can't leave the kids with a babysitter because of things their son has done while being babysat in the past. It's beyond any stranger's ability to deal with especially the teenage girls they used to hire.

So now my husband and I have essentially cut them off too. I feel so guilty about this knowing how little social support they've already got. They both understood we have to protect our daughter, but it was clear they're near a breaking point. He's destroying their lives and they often don't feel safe in their own home.

I've seen them once since this went down while my husband stayed home with our baby, this was a couple nights ago. We ate dinner where their son was remarkably well behaved the whole time, then when we were saying goodbye he whispered "I wish you'd brought (Baby), her screams are adorable." I was too stunned to say anything and he just stood there with a shit eating smirk. My brother sounded totally defeated later when I mentioned it. They were just relieved he hadn't acted out during dinner.

What on earth can we do to help? Has anyone ever known a kid like this and did you ever find any way to deal with them? This seems far beyond normal behavior problems, and nothing works.

TL;DR: I can't do visits with my brother's family anymore because his son is out of control and will try to hurt my infant daughter. This is not my brother and SIL's fault and my husband and I want to find some way to help them.