My Sister Cheated on Her Husband

My sister recently cheated on her husband. She and her husband have been together for 14 years (dating and marriage). I found out about my sister's affair because her husband confronted her publically about it when he found her phone. My sister recently mentioned that she's contemplating suicide which scared me.

Anyway, I'm at a loss of how to be supportive for my sister and help her during this difficult time because i was cheated on myself. As a result, i really really feel for my brother-in-law, but i know this is my sister of course. I'm really concerned for her oldest child who is 10. The baby is barely 3. Anyway, my nephew has been crying over the whole thing.

How do i support my sister even though i dont approve of what she did? How do i help my brother-in-law without appearing to take sides? How can i cheer up my little nephew?

Very conflicted. Cant imagine how my sister is feeling.