The 20 year old guy committed a crime. It is a crime because there is no way for a 20 year old to have a healthy relationship with a 13 year old. It is damaging when adults have sexual relationships with kids.
So now they are 18 and 25? It is no longer statuatory rape. It is still pretty unhealthy.
You make your own decisions as an adult. I hope you can make decisions that will lead to a happy life in the long term. I don't think they should be together.
Honestly the cousin part doesn't bother me nearly as much as the fact that this adult had sex with a child.