Question about Sai?

Okay so in eps 36-37 when Sakura looked out her window and saw Sai on that little bridge thing and said "Could he be.." and then closed her door and opened up her robe and looked at herself, what does that mean? does that mean something about Sai's gender? if not, what does it mean? why does Sai wear such short shirts? also, can someone please tell me what that little book of his is and who is brother is. just from the cover of it, his brother kind of, to me, looked a bit like kimimaro (that guy who gaara and lee fought in the sasuke retrieval arc). i really need help, there are a lot of things that are unclear to me. sorry to be bombarding you guys with so many questions. thank you to everyone who answers, i really appreciate it.