Well, it has to.

“And remember when you kept feeding me so I would get fatter?” Kary asks as she grinds against your butt. “Did you think that it wouldn’t make its way to my genitals?”

“I-ugh-was hoping it would! But-“

“Yes, your butt wasn’t ready for more, but if your hips break or dislocate or whatever, I’m sure you will heal to be much more flexible!”

“W-oww-What?!? Do you really think that will happen? Ooohhh-!”

You feel a pop as your bones lose the battle with the big bone in your bottom. However, rather than the shooting pain you braced against, you feel pleasure. Looking down, your hips doubled in width, allowing your shemale sister’s cock to grow unhindered. Much like your previous growth spurt, Kary’s seems almost entirely confined to optimizing your anal coupling. She actually doesn’t stop growing wider until you feel a bit stretched again! Her length barely grew at all!

“Look at that! We are in sync now like only a brother and sister could be! Well, incestuous shemale brother and sister having anal sex, anyways!”

“This is great! I feel so full of your dick, Kary!”