Anime of late has been plagued by an endless supply of little sisters falling in love with the big brothers, or vice versa, creating the whole imouto craze (or curse) that's going on, so I'm thankful Sugu never had a chance. Siblings falling in love with each other, unknowingly, is an ancient element in drama and it's used for tragedy. Leave it to the Japanese to make serious romance and romantic comedy out of it. As much as I blame SAO's author for being immature and inexperienced, at the very least I give him props for making Kirito clearly reject his sister as well as Sugu not actively chasing after him.