Is repentance considered sincere if what was taken from the deceased husband without his knowledge is given as an ongoing charity and supplication for him, knowing that the taking was sometimes out of defiance, not need, and that the heirs cannot be reached, and is giving charity sufficient to compensate for that?

Whatever a wife takes from her husband's money secretly, without dire necessity, is considered unlawful money. She must return it to him during his lifetime, or return whatever exceeds her share of inheritance to his heirs after his death. If she is unable to deliver the money to them, she must give it as charity on their behalf, and the reward of the charity will be for them. If she is able to contact the heirs later and informs them of the matter, and they approve the charity, then all praise is due to Allah. But if they do not approve it, she must return the money to them, and the reward of the charity will be for her.