What a senseless tragedy. My condolences to your family and those two boys.
I admire your generosity and desire to do what is right for the two children who are so in need, but it sounds like you have some difficult times ahead. Does your wife feel the same way that you do about taking the kids in? Even if she does, I think you are the one in the better position to take stock of the effect that this is going to have on her and your family right now. After all, you say she was already feeling somewhat depressed and now add in the personal loss of her sister and the stress of having to care for two troubled and traumatized kids.
You are going to have to consider long and hard the overall affect this is going to have on her mental and emotional health and ability to cope with the situation first, then what it will mean for the future well-being of your family, and then what kind of assistance you can provide those children with to help them cope with this tragedy.
I'm sure you will be able to provide them with a loving home, support, security, and a sense of family, but it sounds like they are going to need some professional counseling as well. Maybe the school district can help you with that. They have counselors, in many areas school psychologists, and can usually provide grief counseling.
It's a difficult situation for all of you, but you sound like a responsible, caring, and strong person who wants to do what's right for your family and those in need, so you're off to the best start possible. Good luck to you and your family now and for the future.
Alexandra