Advantages:

1 - Potentially easier to get the relationship, because it's already built. At the same time harder though since it's... your sibling.

Disavantages:

1 - Pregnancy is way more destructive, guaranteed to have a child with disabilities. Marriage is possibly way more divisive in the whole family than if you did gender surgery. Hell having a episode of beating your wife would have less controversy.

2 - One of the only ways to destroy the family relationship if the incest relationship doesn't work. Generally if a relationship between 2 people doesn't work you go back to your family since that's the primal one. If the incest doesn't work... you still live with that person for the rest of your life. Awkward.

3 - Talking of the first advantage, because it is easier, this also makes it so you don't have to develop conversational skills with the other gender, potentially ruinning your career if you settle for just your in-law instead of trying to snatch someone in the real world.

4 - Destroy the family bond you have. EG if a mother has sex with son you destroy the mother bond, which could have disastrous consequences.