Eh...so long as it isn't incestuous, a person should marry whatever consenting adult unmarried person s/he deems proper.
What's wrong with incestuous marriages?
There's some level at which it is difficult to disentangle familial power relationships to get a valid statement of consent. Cousin marriages are clearly fine. Sibling marriages are a gray zone..
Parent-child and aunt/uncle-niece/nephew marriages strike me as being too likely to have family coercion in there to have legally valid consent. If I had to draw the line, it would be to ban cross-generational incestuous relationships. Even there, some strangeness goes on, since aunts and uncles can be of a similar age to nieces and nephews, whereas siblings can be very far apart in age or have a power relationship stronger than anything cross-generational in some rare circumstances.
Ultimately, it's about consent, and how certain you can be that consent is legitimate. I would like to say that case-by-case evaluations would solve the problem, but that's too arbitrary for the law to accept.
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