Not to be all "uhm, ackshually!", but since it's come up often enough in manga i read i've actually looked it up before xD
And the only thing prohibited by law is marriage between (half- and full-) siblings. There are no laws for dating anyone afaik. Even for sex, as long as it's consensual (and doesn't break any other laws like age of consent, etc.) it's perfectly legal for siblings to do.
Not sure what the rules/requirements for the civil union stuff is, since those are not legally binding and kinda handled differently in every municipality that has 'em. But I'd imagine they'd use the same degree of kinship as legal marriage for those, if it comes up.