Japanese has a trope where two persons become very close as almost siblings, but later become lovers. Yesterday I've seeing a chart from Bleach were childhood friends became lovers.This could be pretty much the case.
This is one of the tropes:
http://tvtropes.org/pmwiki/pmwiki.php/Main/ChildhoodFriendRomance
This is one of the tropes:
http://tvtropes.org/pmwiki/pmwiki.php/Main/ChildhoodFriendRomance