Is saying "it's normal" when my sister's hand appears in a picture, while knowing it is forbidden and believing she will approve of it, considered making the unlawful lawful or disbelief, and will I be held accountable for her saying "it's normal"?

If the hand means what is above the palm, including the forearm and upper arm, then this is an awrah that must be covered by scholarly consensus, and a woman is sinful for exposing it. However, if what is meant by the hand is only the palm, then this issue has a well-known difference of opinion. Your sister's belief that it is permissible to uncover her hands is not making lawful what is forbidden if she is following a scholar who issues such a fatwa. An ignorant person should be taught.