Going only off this post for context:

I've learned that depression will make me retroactively look at my past and keep wondering if I could have done anything different, and that the fact that I didn't do things differently means that I wasted all that time. It's been an uphill battle figuring out to let go of that and acknowledge the past as it is. I know, for me at least, accepting that hasn't been easy and I still need to work on it. I can't speak to your experience nor make any judgements for it as we are obviously different people with different histories.

However, ultimate responsibility for your nephew's well-being is 99% on your brother, assuming he feels any remorse to begin with, and your nephew's mother.

You're asking what if, the next thing which you probably are already thinking of is what now? That, I have no clue, and will end up being whatever is most reasonable for you and your family. Your parents are supportive so they might be willing to discuss next steps in dealing with your brother. I think it's important to discuss with the rest of your family as well about intervening steps to cut your brother's access to the family, his children more so. Given that your nephew committed suicide already means that the home environment probably isn't ideal for the other kids and they might not have enough time between now and whenever they're 18 to keep dealing with it.

Assuming your brother isn't a lost cause it might be beneficial to have an intervention or see if you can convince him to seek psychiatric and therapeutic help.

If not, then getting him as far as possible from everyone else is probably a potential next step to minimize the amount of damage he is able to inflict.