If the tracking shows delivered, eBay & PayPal should side with you. This is a fairly common scam eBay buyers try to pull these days. Stick to your guns. Inform the buyer that the tracking shows delivered and if it was in fact stolen off her porch, she will need to file a police report. Inform her that mail theft is considered a federal offense and insist she follow up with the local authorities. In some instances, when a dishonest buyer gets the message that you are familiar with the laws regarding mail theft, their package will magically be found, and they will drop the case.
On the flip side, mail theft is more common around the holidays so it is possible the buyer is being honest. Either way, this problem is between the buyer, her local post office, and the police.
Calling eBay to explain your situation may also help to move your case along quicker and release the hold on your account. Best of luck to you!!
Edit to add:
For what it's worth, I have had more problems selling Littlest Pet Shop on eBay than any other type of item. Multiple cases of "my kid bought this without my permission, please cancel the transaction", non-payers on auctions, and other hassles. Many collectors are kids or teens, rather than adults. My gut instinct is that your buyer bought something they really couldn't afford & they are trying to get you to refund it with this "missing package" story.
Call eBay & state your case. Don't let the buyer pressure you into a refund.