If you consider 12 "of breeding age," you're sick..

Guess what pal, any girl that has had her first period is capable of having a child. It's not wrong for a boy a few years older than her to take notice of her. You are raging against the natural world and labeling unnatural. You are the one who is wrong here, not human nature.

Plus, the OP has said his sister has not gone through puberty yet. You condoning his cousin's sick actions is deplorable, and frankly scary. The age of consent in most places is around 16, not 12, for a reason

Uhhh... to consent to have sex with someone older than the age of consent. If both are underage, it's not statutory and most places have laws specifying that an of-age teenager and an under-age teenager are not breaking the law if they're within a few years of each other.

The OP has said his sister hasn't gone through puberty yet but at 13 that's very unlikely - certainly she is not post-pubescent but she's almost guaranteed to be pubescent, else it's about two orders of magnitude deviation. The OP has also been doing everything in his power to twist this situation into a worrisome one when its clear that the two parties involved and all the adults who know about it do not have an issue.

Why are you scared of teenagers having sex? Weirdo.