I too am sorry for your loss but know you can take comfort in knowing that she is in a far better place, and that you will one day be reunited.
I was once a foster child, and the testimony of your former foster son really struck home. It takes special people to be good foster parents. The challenges are many and the system, and very often the child, don't make it any easier. To see such a glowing testimony speaks volumes about your wife.
I am sure she is resting in the loving peace of our Savior's arms.