Why not the protagonist’s own grandmother? Because the "wife’s grandmother" carries a unique combination of safe distance and familiar intimacy .
Her secret to staying young at heart is perhaps her passion for lifelong learning. She is always eager to try new things, whether it's learning a new recipe, taking up a new hobby, or simply staying up-to-date with current events. This curiosity and openness to new experiences have helped her stay engaged and motivated, even in her golden years. Tsuma no Sobo wa- Mada Mada Gen-eki Chou Bijuku...
(妻の祖母はまだまだ現役で、超美熟…) Translation: "My wife's grandmother is still very active/vital, and she is a super beautiful mature woman..." Why not the protagonist’s own grandmother
Focuses on a relationship between a younger man and his wife's grandmother, a common trope in this specific niche of Japanese adult media. whether it's learning a new recipe