Hi Dbh,
I'm running out the door but wanted to post quickly. (I also lost a sister to suicide). I'm so sorry for your pain. I wish I could hug you or offer magical words that would make it go away.
This is going to sound callous and totally terrible, but my advice to you right now is to put yourself first. My family has so many problems, had I not learned to do that, I would have sunk along with their ship. It doesn't mean you can't care about your BiL and nephew, it just means that when it starts to feel overwhelming and like it is bringing you down, you must detach, take a break from it, and find some happiness in your own life.
There are a lot of people on here who will know better than I do what to do about your nephew but I personally would NOT call CPS. I can not see how that would help your nephew in any way whatsoever unless he is being physically abused.
My sister was angry with me when she died too. When I feel guilty about it, I imagine her as a spirit, free of her mental illness, and I know in my heart that the healthy her loves me and does not blame me for anything. They are not themselves when they commit suicide. They are mentally ill, their brains hijacked by depression. Please try to focus on the memory of your sister before she got sick and let the memory of the sick person fade away. She is free of that person now, you too can let her go. I know my sister loved me and I know she knew I loved her. I'm sure it's the same with you and your sister. The rest is just disease like a bunch of mucus on some kleenex. Throw it away in the trash.
Do something for yourself today. Go to church and pray, go on a walk--nature helps so much, meet a happy friend for lunch, watch a funny movie, or whatever lifts your sprits.
We care about you and are here for you!!!