Responses
I think you're wrong xd
It's their parents' marriage
Then their relationship with each other would be legal and therefore, be officially siblings.... and most likely couldn't be together. There's a manga where the couple have to get married first before their parents get married. But, who knows? I don't know if siblings, blood -related or not, can legally marry.
Their names weren't entered in the family register, so I'm pretty sure they can still get married
Wow ! They actually got married while still in high school. Iroha was asked if she would keep her last name... and she said she would , since it would be a problem as she was still in high school, and the fact that she started high school with her last name Tachibana, it would then be a bit troublesome to change it halfway through high school.
I guess, the couple couldn't contain their " physical love" anymore for each other so parents agreed , oh, heck, just get married already before you two commit a sin, he he. I ;m sure that's what happened.