Are you keeping the grandparents in to limit births or do they serve another purpose? Looking forward to meeting Reiko's nooboo!
There are several reasons.
1) Limiting births: I harbor the ungrounded notion that I have a better chance of a perfect genetics heir with 2 sets of twins, rather than triplets and a single birth.
2) Prevent pregnancy-by-abduction: My sims still get abducted every few weeks. I'd rather not have to "Prevent abductions" via the satellite dish every 24 hours.
3) Every heir couple ends up completing Big Happy Family, since they have 3-4 children before a "perfect genetics" heir is born.
I like to keep one parent around to help mentor skills, particularly for a move-in spouse.
4) Magical training: Every heir reaches max magical rank then stays long enough to help the next heir do the same. The current heir, Reiko, has 1 or 2 more ranks to go.
5) If I particularly love a certain sim, I'll keep them around as long as possible just cuz.