That's fine -if you don't have a package and have a room only reservation just add her now to your reservation for the entire trip.
If you do you have a package (everyone has to have the same entitlements) then just wait until you arrive and add her after you check in. She will have to get her park tickets separately and pay for her meals if the rest of the party has the meal plan. This will also mean she will have to wait till 30 days to make FP+ reservations but with the exeption of a couple attractions you should be able to copy yours to her.
If you are room only and can add her to your reservation now just make her DME reservation based on her flights and dates separately from yours now as well. If you have a package you can make her DME arrangements when you add her to the reservation but she won't have the luggage tags for the auto luggage delivery.
We have done this many times (as recently as last week!) with my adult dd's and my sister but I always book room only.
