For one, it's just been done to death, especially in incest games.
The other reason would be that most adult women aren't into that androgynous little shit type of guy.
They want guys who actually look and behave like grown-ups, not little boys.
It pretty much has become a pattern:
Mother is tall (5-10 or so), taller than MC
Older Sister is as tall as Mom, but a sporty figure compared to Mom's lush one.
Younger/Twin Sister is short
Aunt is tallest yet, usually hitting six feet or more.
and MC is short, for a male.
Basically, age is represented by height. And that's fine! Creators are gonna create their vision, and I'm thankful for their work. But if someone wants to differentiate their game, it would be worthwhile to look around. And I may well be wrong in my limited exposure, but it seems I'm not the only one. It could actually be refreshing to see some models portrayed in different views (I'm pretty sure the same aunt is used in, like, 43% of games!).
Anyway, this looks interesting and I'll keep my attention on it.