I have been dealing with anxiety mixing with attention defesit disorder and it makes my brain puzzled and makes me frustraighted. So maybee your brother in law was very puzzled and frustraighted inside. I think alot of times suicides feal trapped and that there is no escape. I believe sister should understand that it was not her fault and that God knows what was wrong with him and that he made a mistake i believe God will forgive him and i think she should find the strength to forgive him. Your brief storey helped me out because i have been thinking about commiting suicide but i always have to keep in mind when you end your own life it affects everyone around you and that keeps me from ever following through. If she wants to know why she should talk to other people who have had a suicide in there family. I know several and if she wants some advice from them i'll have the write something in the message board.
I Just Want To say I`m so sorry,it will be hard for your sister,I wish you all well.