Hello, Rachel!
A trip to Walt Disney World with both you and sister there at the same will be truly spectacular! What a treat!
We just returned from a multi-generational trip to WDW where we had purchased Disney's PhotoPass + and really loved it! The bottom line here is that you, as the purchaser, can add as many Disney PhotoPasses to your account as you'd like. We had multiple PhotoPasses throughout the trip, since there were so many of us, and we sometimes neglected to bring our PhotoPass from the day prior with us to the parks the next day. No worries! We simply added all of them to our account when we arrived back home.
The only caution I'd give you is that you'll need to have the PhotoPass + card with you when you'd like to add attraction photos. You can add multiple attraction photos, however, since there may be so many of you in your group that you can't fit in one car. :-) Again, no worries, just add everyone's photos from your party to your PhotoPass + card.
On the other hand, if you would each like your own PhotoPass + Card, you can certainly do so. As with any other PhotoPass Card, more than one Cardholder can have a digital photo attached to his her card; on the Moms Panel, we've done this frequently, as there may be as many as 20 of us with our own cards, each of whom wants to capture the magic of the moment for later viewing.
Either way, be sure to take advantage of the
Disney PhotoPass + Pre-Order Discount, so that you'll save $30 on this already great deal.
Have a fabulous trip! Be sure to send more questions as you think of them, too! We love to help!