Well in this instance I would say once an offender always an offender. I'm assuming the cousin is an adult here, and therefore if you're an adult you know better-PERIOD.
When you get to a 16 or 17 year old with an 18 year old and parents get involved because they're "shocked" their DD or DS was having sex and they press charges, not so much. But if you're an adult and you're having sex with a child (yes 18 is technically an adult folks I know but come on) then you're not going to change.
I think the younger the child is that a pedophile goes after the sicker they must be. There is no excuse for damaging a child this way. It's so wrong. I think there is a big difference between a 16, 17, 18 and 19 year old but they all have immaturity on their side. Once you get into your 20's IN GENERAL most people aren't so stupid and know what is right and wrong. Not that 16-19 don't know, and their age isn't an excuse but I think sometimes their lack of experience and age causes them to not think.
In this case though any adult knows not to touch a 13 year old child. She didn't serve enough time in my opinion, and my kids would not be around her at all. If DH couldn't get on board that would be tough sit in my opinion. This is your kids safety.