I'm going to threaten to tell my sister's secrets.
That's not kind, why would you tell her secrets?
I know some serious secrets and I have never told anyone. If she continues to blackmail me then I might reveal them.
Siblings should get along with another. If you're being blackmailed then telling their secrets will only make things worse. While your relationship seems to already be bad, it may be salvageable if you don't escalate the situation.
She's blackmailing me, I'm going to use the secrets to hurt her.
It's not good to betray your sister's trust as you don't know how she is going to retaliate against you. If she's blackmailing you then she can hurt you in the same way. Your relationship sounds very toxic and I would highly consider family counseling.