This goes off so quickly in such strange and awful directions o_o; I couldn't make it past five chapters with all the incest jokes. Was not prepared.
Also the idea that they call each other "grandma" and "grandpa" most of the time, even when alone together, was just about distractingly strange—though I'm sure there are indeed old couples who call each other in such a manner for some reason or another.
@FRASH_18:
In the first place if you're implying that old people think about sex less—that's not really how it works. Frequently "again" wouldn't be the operative word for "horny", either, to my understanding. XD
So, well, yeah, IRL it does seem rather unlikely that a suddenly rejuvenated, comfortable happily-married couple (With kids! Which came from somewhere!) of this sort would be at all shy about putting their newfound vigor to task—only caveat I can think of would be being uncomfortable about the sudden body changes in their own right, but that doesn't seem to apply here.