It’s direct and won’t require you to rely of disguises or hiding.... maybe... but it is risky. Getting in as the slumber party occurs is dangerous and there is a high risk that you’ll be spotted, but at least it gives them less time to notice something is off.
First of all you could deliver them food, maybe even offer to be your sister’s courier and hope you’ll be able to pick up some chuncks of information in the brief time you’ll be able to get in. You might even be able to slip in if whatever you give them is distracting enough.
Then of course you could head in through the window. Maybe when they’re watching a movie or something. Climbing in and then finding someplace to hide while they’re all distracted.
Then there is the Trojan horse solution. You could hide in a box outside your sister’s room and hope she takes,the bait and brings you in. Maybe she’ll even put off opening it long enough for you to listen in. It’s a stupid idea but worse ideas have worked in history.