Technically, it's a "cousin crushes/wanting to kiss each other" thing, but cousin marriage - a cousin love triangle, actually - is a apparently a major plot point in the sequel.

I did grow up in a big, close-knit family of cousins, so... yeah, no, that is deeply weird to me. On the other hand— I don't remember being as weirded out in the case of, say, Nikolai and Sonya in War & Peace? Hmm. (Then again, I think I was distracted by the feeling that Nikolai and Sonya had a much bigger relationship hurdle on their hands, namely that Nikolai.............. did not strike me as exceedingly heterosexual.)