What is the ruling on a sister taking her husband’s trust (amanah) without his knowledge, and what is the legal ruling on helping her return the trust to her father? And what should be done in this case?

What the sister did is a transgression against her husband's trust, and she must return it, repent, seek forgiveness, and not inform her husband. There is no sin upon the questioner because "no bearer of burdens will bear the burden of another," but she should advise her sister to return the trust, repent, and seek forgiveness.