Harav,
I will give names, so it will be easier for me to phrase my question: Let's say a man his name is Yoash, and his mother is Naomi, and his wife is Ruth. Now, let's say that Yoash dies, and after he dies Naomi has a son. To my understanding of the Masechet, the new born son cannot be a Yabam, at least for two reasons:
1. He was not in this world when Yoash was alive
2. He is his brother from the mohter only.
If this is the case, why in Megilat Ruth Naomi tells Ruth and Orpa not to wait for her to have children? of course they cannot wait, they SHOULD NOT, because according to the masechet, they will not be able to marry him.
Thanks,
Aaron
The Medrash Rabah on Rus (quoted by the Torah Temimah on Rus 1:11) in fact states that Naomi's question "ha'Od Li Banim b'Mei'ai v'Hayu Lachem l'Anashim" -- "have I anymore children in my innards and they will be for you for men?" (Rus 1:11) was a rhetorical question concerning Yibum (as well as her inability to bear children). The Medrash describes her thoughts: "Does a person do Yibum with Eishes Achiv Shelo Hayah b'Olamo?" The Ibn Ezra similarly explains that Naomi's statement was not that they shouldn't wait for Yibum, as that would be impossible due to the children's being brothers from the mother's side. It was rather a loving and sympathetic statement that if she could produce children which would take the place of their husbands, in order to help them, she would. She knew this was impossible both from a practical and halachic aspect, and sympathized with them in this fashion.
All the best,
Yaakov Montrose