For those confused, I think it may help to look at it this way. One could say, "Wir werden mal sehen was passiert." (We'll see what happens.) But it is common practice to just leave out that first part and say, "Mal sehen [whatever]" or "Mal schauen [whatever]". Sometimes, people even just end the sentence there, i.e. "Mal schauen." (We'll see.)
You're right, in this case, "werden" is not a modal verb but rather a helping verb; in terms of conjugation, however, it functions the same. I don't really get your question, though. What sentence are you referring to? In "Mal sehen was passiert", there is no "werden". Even if by "sie" you mean "Sie", I'm still a little lost.
A shot in the dark: if you're asking why the translation is "let's see" / we'll see", rather than "she'll see", "you'll see", or something else, I'd say that just like in English, those are pretty rare. (Other meanings of "you'll see", e.g. "I'll prove you wrong" ("Du wirst schon sehen"), would be translated differently or explained by their context.)