Are you saying that the brother who has to perform Yibum must be born prior to the marriage of his sister in law?
I recently heard of a case where a man died childless. He had no brothers, but his mother was pregnant when he died. When she gave birth to a boy, the wife was obliged to wait for 13 years to get Chalitza from her brother in law who was born AFTER her husband died. Does this make sense?