[aaronbrethorst]

You can't marry any sibling, regardless of gender. You can marry some people, but it's gender-dependent. It's called equal protection for a reason.

[cninja]

Both gender and relationship to the potential spouse are attributes of a person that cannot be changed. Why is it ok to discriminate based on one but not the other?

[aaronbrethorst]

That's irrelevant. No one can marry a sibling. Anyone can marry the opposite gender.