I had a co-worker who used to stay with her dad in NJ during summer breaks (she resides in FL), and she was able to work at a starbucks in NJ for those months and then come back to her home store. You just have to set it up with both managers, which they might have to get it approved by their DMs.

Edit: you could also just pick up shifts whenever you feel like it. Maybe leave your number in their backroom at the store your hoping to pick up shifts from.

If you can find a store that's okay with it then you can probably do it easily. People transfer back home for the summer from their college stores frequently. All you need to do is talk to your manager and make sure they are okay with the quick turnaround and then start calling stores near your sister to find one that is willing to take you. Make sure you give each manager at least a month to get the transfer done properly.