Wait, I made a mistake. Obviously it cannot be mechayev his sister in law in yibbum- that is eishes ach shelo haya b’olamo. So the only question is if it can pattur his father’s wife from yibbum.
So for my sfeik sfeika case, we’ll say this:
Suppose there was a son, but the father and son died in a collapsed building and it is a safek which one was first. And, now there is another safek if there will be another kid conceived later.