Sei & Shimako Soeur relationship

At first glance, you'd think that these two people are so distant you'd wonder why they even agreed to be soeurs. Sei's a big flirt and Shimako's so polite to everyone. And then, before I can conclude their relationship is bland like water, the question of why Shimako rejected Sachiko for Sei is still unanswered. Shimako said Sei has something to give her and vice versa, and the series doesn't tell us what it is, but rather, show us. This pair ain't as married as the yoshino-rei pair, nor are they like the honeymoon pair that is Sachiko-Yumi. They're different. They are close, and they care for each other deeply (I'm unsure abt Sei, but I know Shimako does, as proven in episode6 where she firmly rejects Shizuka's offer, claiming that Sei will be her only soeur. Her rejection was spoken with such intensity I was surprised she had it in her!). Their relationship have an intangible emotion lingering around, and I was pleasantly surprised with the frequent mentioning and slow revelation of the depth of their relationship. Hehe this is one relationship that I'd wish will develop further than friendship...maybe into the arena of ... love? (shoujo-ai type mind you) Bcos it's so obvious they're way deeper than plain friendship...