If a man has two wives, the one beloved and the one hated, and they bear him sons, the beloved and the hated, and the firstborn son is the hated one’s—
English: “And the hated” — to include the forbidden relations for which one is liable to kares at the hands of Heaven.
On the daf / in context: Building on the earlier teaching that kiddushin takes effect with women forbidden by negative commandments, the extra “ve-ha-senu’ah” includes even cases of kares. The inheritance consequences still apply.
Type of Learning
Ribbui from the extra “ve-ha-senu’ah.”
What Is Learned
The inheritance rule of this section extends even to unions that incur kares; the child of such a union may still be the firstborn for inheritance purposes.