Is it permissible for a person to sign a marriage contract on behalf of his sister without a legitimate power of attorney, and does this entail expiation?

If your sister married legally outside her country, and you signed the contract impersonating her, then you have fallen into falsehood and deception, and falsehood leads to immorality and Hellfire. If the marriage contract was false for the purpose of travel, then both of you have committed a sin. The expiation for this is sincere repentance, by abandoning the act, regretting it, and resolving not to return to it, in addition to much seeking of forgiveness.