Merely calling a girl sister does not maker her one but it does create a sort of implied barrier between two parties to extra intimacy (to some extent at least). However, this does not mean that frankness with that girl should be of the same level as with one’s biological sister. Only that kind of interaction is allowed where norms of gender interaction as mentioned in Quran can be observed properly (Please see: Discussion 30301). It is more likely than not that a boy and girl talking in that way would at some point deviate from those norms, so a conscious muslim should be highly cautious about such interactions.