You have misunderstood what a few people have said.
Your family IS dysfunctional - not you/your kids, but your sister and her kids, you and your sister and your parents etc, Somewhere in all this the dynamic isn't healthy and while this is not up for discussion and we are not right to judge because we have no information, the fact that there is a rift there shows that there is trouble in relationships/family around you.
Your Nephew is expressing the fact that he feels suicidal, this is not to be taken lightly, and while you have 4 kids and are run off your feet (like I wouldn't even imagine) you have a young person under your roof that needs help.
ALL you need to do is to call the school and tell them what you have told us. You should call the Dr too if you can, but the school will be the best first step, (a) because they will want to know what is going on from an attendance point of view and (b) because they have systems and processes in place to help protect kids who seem to be struggling mentally.
Social Services SHOULD be involved tbh, not with YOU, for HIM. To help HIM manage life. Your DN needs help, he is under your roof, it won't take long to ask someone for help. Your sister clearly isn't going to do anything about it. That's not acceptable