Really tough, that's got to be triggering for you too. Sounds like advice from colleges is a great idea.
I get the desire to encourage healthy relationships, but the way I see it, when it comes to abuse, especially for children, safety has to be prioritized, and your nephew is clearly at risk of harm and is suffering damage. It's really sad.
It's good that his parents are both in therapy. That should help with PTSD. Less likely to help with BPD or NPD, unless the person's gained awareness of their own disorder and they're not just projecting/blaming. My mum's behaviour fitted with NPD, and my dad's with PTSD. My dad was fairly volatile, and sometimes overly critical, but looking back I can see how hard he was trying, and appreciate so much of what he gave me. Really was very caring though angry, depressed, and confused.
Still have lots of resentment for my mother's behaviour. I married a man whose behaviour fits with NPD too, and ended up suffering physical abuse. I really tried to encourage a healthy relationship between him and his son. That always seemed the right thing, but when I had to leave, I had a long talk with my step son. He was sixteen at the time, and really very independent for that age.
I didn't let on how bad the abuse had got, and he really wasn't party to it, but there was a lot of tension and discomfort in our home.
I doubted then whether he was safe with his dad (I hadn't before). I asked if he felt safe, and what he would do if he didn't. He said he was safe, just didn't like his dad's moods, and he would go to his grandma and granda's. He told me that he was angry that I did everything his dad said. I didn't do that, but he might have heard his dad (that day) saying I should leave. He also said his dad was "like a robot". I think his dad was scared of him. I think he was scared of me too, and kept a lot hidden about himself, until I was at my most vulnerable.
It's really impossible to know what's going on in private. I think my step son's mum might have had a personality disorder too. She was at least emotionally immature, badly jealous, and he was often in tears because she had cried to him about how he "loved other people more than her", and things like that. He really was heavily burdened at a young age. I'm sure you know the feeling, which is why you want to help your nephew.
Really would just suggest encouraging him in his own endeavours, and only encouraging him to be safe with his parents, and understand their limits. He shouldn't take on the burden of their suffering at such a young age. Poor kid