It is the better way round, and worth encouraging if she is willing.
What changes
The trial starts the day she signs up. If you sign up first, nothing starts until she joins. If she goes first, the fourteen days begin then.
She invites you, rather than the other way round. You join her Circle.
She sets the sharing from the beginning, category by category, person by person.
Why it lands better
The same tool feels different depending on who set it up.
If she signs up and invites you, this is her app and you are a guest in it. If you sign up and invite her, it is easy for it to feel like something being done to her, however kindly it was meant.
That difference in feeling turns out to matter more than any feature, because it decides whether she keeps using it in month three.
What is the same either way
She always holds her own login and her own account. You never manage a profile on her behalf, and there is no version of this where you can see something she has not chosen to share.
That is true whoever clicked first. Signing up on her behalf would not give you more access, which is worth knowing before anyone tries it as a shortcut.
If she wants help doing it
Sit with her while she does it, rather than doing it for her. Her email, her password, her choices on the sharing screen.
Twenty minutes at the kitchen table is the single best predictor of whether a family is still using this a year later.