Its down to how the payment was SENT to an account.
1. If sent using money > goods / services option then paypal would see the recipient as a seller and apply the new seller hold. They would also charge a fee.
2. If sent using the send money > family / friends option then the recipient would have the payment instantly (as long as it wasn't an echeque pending with a clearance date) with no fee if funded from a bank account or balance (a fee if you use a card). (However that option is not available in every country).
If your sister marks the transaction as processed then you may have the option on your account to mark the transaction as delivery/satisfied. If you confirm this by clicking on the "Confirm Receipt" button, the funds may be released sooner. This is not always guaranteed. If not then sadly its 21 days.
Advice is voluntary. Kudos / Solution appreciated.