Yikes!
So sorry you are dealing w/this OP.
This family should have been upfront w/you about their son.
They probably knew that if they disclosed how he was that he would not have been accepted into the share which I find despicable.
So since they failed to disclose how their son was to you from the get-go, then they do not deserve to be given a two-wk notice period that you will no longer be caring for their child.
Yes - it WILL be uncomfortable + awkward letting them know that you will no longer be caring for their child but knowing that your host family stands behind you may make telling them a bit easier.
I wish you the best.