It is beyond hard. It is impossible to "cope" with any of this. Having said that. I see nothing confusing here. What do you imagine you can possibly do to "handle" any of this wall-to-wall tragedy? Do you imagine yourself organizing an "intervention" to get your nephew to stop drinking? If you are entertaining any such fantasy, I would strongly encourage you to get yourself to some Al-Anon meetings and grab an armful of their literature. This whole mess is not yours to handle.

Consider whether you have knowledge of child abuse that you have a moral obligation to report. If you do, then make a report of what facts you have direct knowledge of. Beyond that, protect yourself and your own children from any enanglements that will only have an unwholesome effect on your lives.

By all means, remember these wretched folk in your daily prayers. Then focus on your own responsibilites. Fixing your nephew is not your job. You probably have zero capacity to affect anything going on with your sister, her son or her grand-daughter. Your grand-niece is in a facility where she is being cared for. Be glad of that. Your nephew is unlikely to get custody of this child you mentioned above. Your sister is not interested in sharing with you what is going on in her family. Be glad of that. It's a bunch of bad news that you can do nothing about.

What can a therapist possibly tell you? I expect any therapist would agree with you that this branch of your extended family is a mess. But it's not your mess. Be very glad about that.