A possible reading of the situation is that Keiko always thought of herself as being heterosexual but the current circumstances have got her entertaining the possibility of trying something more serious with Ran. From a few chapters now Keiko's attitude has been more reminicent of a girlfriend than a friend.
Agreed!!You can Keiko attitudes changing when compared to how she acted towards Ran in the beginning, like being more protective and honest. I also think it is wonderful how we can see she becoming a happier person and it is all thanks to Ran`s influences on her.