For some reason, I think Anastasia was the sacrifice for the new curse. Lady Tremaine sacrificed her own kid to bring down her vengeance on Cinderella and try for a new better life. This will probably make her irredeemable because it would be the darkest thing OUAT has ever done, and personally….I see no way you can redeem someone whose done that.
"If you go as far as you can see...you will then see enough to go even further." - Finn Balor