The older character is extremely horny and trying to get off by themselves when the younger interrupts. Being a sexuap deviant himself, the older invites the other to participate, deciding that what other sex education would be better than sex with someone trusted, experienced, and that already has a strong bond with them? The younger one experiments with fingering, and eventually ends up penetrating the other.
Shota welcomed, but not necessarily required. The younger can be a teenager, but an early teenager if that makes sense. Like 14 or something. AUs welcomed, just ask. Preferably the two live together. Blood relation does not matter.