Sister-in-Law, I'm Really Not Here to Take You All Off Their Hands

I'll take them all.

My big brother is a savior. While saving the world, he also happened to marry hundreds, even thousands, of monster girls. Now that he's on his deathbed, he entrusts me with setting things straight with my sisters-in-law.

"Big Brother, I'll become a woman to prove my resolve. I would never do anything to betray you."

"Little brother, as the old saying goes, a younger brother should inherit his elder brother's wife and children..."

"Mom, who's this auntie?"

"Silly child, she's your uncle. But now she's your dad."