Everyone saying they’re cousins or mentioning incest seem to be forgetting how ancient clans work. The fact that her grandfather said he was a “distant relative” of hers means that their degree of consanguinity most likely exceeds the fifth grade. They probably don’t even share the same last names. Yet they’re still relatives cause all the clan is family regardless of how far apart on the tree they are.
There’s also the second possibility, one other thing Ancient Clans used to do a lot. He’s adopted. I mean, if you’re a martial clan, adopting that unit of a man is only logical right?
He’s the ML, his design couldn’t have made it clearer. Since he’s the ML, he’s definitely the MCs romantic interest.