Oh, so the groom told everyone else first and even had sis picked out the ring setting. If that’s any indication of the marriage...
She knew.
It’s a cute little story, but come on. She knew.
She isn’t dumb. Not by a long shot.
My guess is, she played along. Not wanting to spoil it for her fiance.
My husband’s niece told her sisters, and some of us first because he was looking to buy a ring for her and needed advice. The sisters guided him as to what to buy, I went and bought it since they live out of the country, and my sister took it down.
Asking the parents is the traditional, gentlemanly way of doing it.