P.G.
She's not ready - it may come off as a game with the sitter so she doesn't feel pressure, but it doesn't seem to be working at home, so don't worry about it. Honestly, if a kid can't pull their own underwear up/down by themselves, it isn't really them that's potty trained, it's the adults.
Have the potty around if she's interested, bring her to the bathroom with you. Let her show you when she's ready to do it at home, but don't push it. Ask your sitter to talk to her about pottying at home just like she does at the sitters so she can see the connection.
Don't be frustrated - she won't be going to college in diapers. It's her body and it's really the only think little kids have any control over at all. Don't make it a battle.