More precisely, it's only Miyuki who wants to fuck, Tatsuya can't horny.

+1
Most likely, he saved her life or some BS like that (The usual cliché)... and that's not even a good explanation, because there is none for 2 siblings who love each other romantically. It's just weird and f**d up.

oh but I do.... >:) Wincest

But they're cousins :(