I could be slightly off,
but I think I read their ability to empathasis were manipulated on the genetic level before they were 'born', in order to make sure they could only fall in love with each other.
This developed naturally, as you can see in the prequel arc where initially Miyuki didn't like Tatsuya and he just wanted to be a good servant to her. But after he brought her back to life, Miyuki slowly grew more attached to him and couldn't help it as she started to see him as a man. Like Tatsuya is saying here, he accept the marriage engagement but at the point he still sees her as a little sister.