No it isn't an issue. I've played it and the sister with the green hair looks to be about or older than 15 years of age. Merriam-Webster dictionary defines puberty as "the age at or period during which the body of a boy or girl matures and becomes capable of reproducing." In order to reproduce, the girls need breasts to feed a baby, a wider hip and matured vaginal area for childbirth. As for the guys, it's facial and body hair, broader chests, testes descended, larger penis, and more muscle development. As long as the characters in sexual situations show most of those signs and look older than about 12 years of age, it is allowed.