Well at least you have options now, and I would be thinking the best way forward is to put yourself in a position where you have options down the line.
First thing I would do is get stalks off the dog.
I would put the brother to the sister and if there is a proven bitch of a similar type that you have access to, I would put the brother over her. You have options then with father to daughter, son to aunty etc.
Other way would be to get a proven dog of a similar type to put to the sister and breed the brother to the pups, but for me the first option is the better way if you can