So it's only been three weeks since my sister died.....my friend who is getting divorced (still living with and married to her husband) has been helping my brother in law with his new house (I introduced them and encouraged the working relationship).
He told me he asked her out after the divorce is final....since then they have been communicating via text. My friend hasn't told me anything...so what do I do?
Wow. I am sorry you have to deal with this so soon after the loss of your sister.
The important thing, I think, is to do what you need to do to take care of yourself.
You can't control the actions of others, but you can control what you will or won't be around. Only you can figure out if this dating arrangement is something you can tolerate, or not.
If it is too upsetting to you, don't feel guilty about putting some distance between you and the relationship that is developing between your bro-in-law and your friend. Just tell them plainly that it is too stressful for you, and you can't be around them right now.
On the other hand, if you are feeling like you really need your friend, and you can handle the dating without being overly stressed, then go ahead and maintain your friendship, and don't feel guilty about that, either.
You are dealing with a tragic loss and the pain of bereavement. Think of yourself and be good to yourself.
All my sympathy for your loss.