I'm so very, very sorry for your loss.
Try to resist the urge to "take over" for your brother-in-law. You can extend practical offers to help during this difficult time, but he has the right to raise his own child. You may be surprised at how he steps up to the challenge once he gets past the immediate grief.
You could not have prevented her death. And detachment or distancing does not cause anyone to take their own life. Even if you were close she probably would have hidden it from you.
Don't be so sure that other survivors can't or won't understand. Just as we all understand each other, even though the details of our experience may be different, the same is true of suicide survivors--everyone feels they should have known, or done something differently, all have the same kinds of unanswered questions and regrets.
Hugs, you and your family will be in my thoughts and prayers.