Commentary for Kiddushin 125:2

In this case, a man makes a betrothal statement to a woman at a point at which the statement cannot be fulfilled—one of them is not Jewish, one of them is not free, she is married, he is married to her sister, or she is subject to yibbum. In all of these cases, R. Meir says she will be betrothed when the event occurs. This is like a case of transferring title for something not yet in the world, because at this point she is not permitted to him.
R. Judah Hanasi theoretically allows such a statement to be effective, but says that the rabbis did not allow it because this would create enmity. Imagine how this would play out in family dynamics if he’s married to her sister and he says “Behold you are betrothed to me after your sister dies.” Might make a good TV show.