seohyun is closer to yoona because: 1. they were childhood フレンズ even before debut. 2. their houses r near each other so they can talk and play. 3. yoona is the 秒 youngest which means she can relate to seohyun a bit もっと見る than the others can. 4. Yoona is the one who is the nicest to seohyun. 5. Seohyun gets もっと見る influenced によって yoona than the others since yoona and seohyun talk もっと見る with each other.
For me, its Yoona eonnie, though Hyoyeon eonnie is currently the roommate of SeoHyunnie, we know for a fact that the two are known as childhood フレンズ and they are almost same age. for that, i can conclude that they share common interest and hobby.