Yevamot 17 - A very young brother
The laws of yibum apply only when two brothers live at the same time and one dies childless. However, if the brother is not a contemporary, he cannot do a yibum. How do you mean?
Let us say there are two brothers, Reuven and Shimon, and Reuven dies childless. Shimon should now marry Reuven's wife, but he waits. Meanwhile, a third brother, Levi is born. Now Shimon marries the widow. He has his own wife also, and now he dies - the two wives are looking at Levy. However, Levy cannot marry the first one, because she is his brother's wife, and when Levi was born, Reuven was already not alive. Thus, the first wife goes free and does not need either yibum or chalitzah from Levi. The second wife goes free because of the first one. This is one of the fifteen women of the basic rule , but here we treat it in more details.
How do we know that the above is true? - Because the Torah said, " If two brother live together (in the world) " - which means that the two brothers should be contemporaries, and then the laws of yibum apply. If not, the other brother's wife is always forbidden to the young brother.
Incidentally, this phrase also teaches us that it applies only to brothers of the same father - just like the twelve brothers who were the sons of Jacob and are called brothers - so here too, "brothers" should be from the same father.
Art: Jacob Sending his Son Joseph to Look for his Brothers by Eustache Le Sueur