Firstly, so what?
Its his sister, who is a woman. I don't know what your views are to incest, Whatisthis, but at the end of the day, the probability that they will have a handicapped child will increase slightly. Nothing else.
Their eventual neighbors may gossip that they look uncannily alike, and their parents may never want to see the couple again, but otherwise its a healthy, heteroloveual relationship.
Furthermore...
You're the most gullible person ever Whatisthis, lol
The
*actual* probability of someone:
a) Opting to marry their sister and,
b) Announcing it to random people on the interwebs...
...is unlikely.
