[Assuming she's being allowed to talk again once she quiets down.]

The only sister I have is named Sweetie Bell, and as far as I know, she's still safe back home in Ponyville.

[Slowly standing up, she shakes back and forth a bit in an attempt to knock off some of the slime.]

Well, then, there's only one thing to do in a situation like this. Take a long bath, and then deal with the impostor.