I'm sure it'll take a fair bit longer from this point for her to act on this, if she's the one to act on it first at all. There's just too many things she can focus on as problems between them in that regard. One, if she asked him out and he said no because his sister was mistaken she just made things awkward between them and they still have to see each other every day at work, that'd be pretty brutal. Two, since that one would just be in her imagination as the dude is totally in lurv anyway, at the very least they need to clear this with HR assuming the company even allows this kind of thing at all. Some might not. Three, I seem to recall they have an age gap of some amount going on here, even if both of them are more or less okay with that she might worry about how people will see her, him, or both of them.
I can probably come up with more but I don't feel like it right now. Point is, both of them even being completely aware of their feelings for each other? That's like, step one on a multi-step path to even reaching a confession from one or both of them, I'm sure.