Hi Chong! Thanks for dropping by planDisney.
How exciting that your sister is coming for a special visit. I'm sure you're looking forward to seeing her and spending time as a family. A visit to the parks together certainly would be a magical way to spend the day, and you do have options. I'm happy to walk you through them.
Guests currently need a valid admission ticket and theme park reservation to enter the parks. On popular days it is possible for parks or days to fill up. I checked the
theme park reservation availability calendar, and it looks like there's current availability for
Disney California Adventure Park on your preferred date. This could change quickly, so if you're interested in modifying your existing park reservation and purchasing an
admission ticket for your sister, you'll need to do so soon. To modify your park reservation from Disneyland Park to Disney California Adventure Park, hover over "My Disneyland" on the Disneyland Resort website and select "Ticket and Reservation Details." Scroll down to "Make or Modify Park Reservations" and click to begin the process of switching your park reservations to the currently available park. Then, be sure you select Disney California Adventure Park when purchasing the new admission ticket for your sister. You don't want to end up in different parks!
Alternatively, you can keep checking the theme park availability calendar. Guests' plans do change and if someone cancels their reservation for your preferred date and park, you might see availability open up again at Disneyland Park. There's no guarantee, but sometimes persistence will pay off and you can grab an admission ticket and reservation at the last minute.
I love Disney California Adventure Park. I think you'd have a fantastic time there together. The Attractions are thrilling, the dining choices delicious, and Cars Land has my favorite theming of any Disney park. If you want to be certain you can go with your sister, I'd modify your existing reservation and go with that!
I hope this helps.
Elizabeth