What should the questioner do to absolve herself after stealing jewelry that her deceased sister entrusted to her until her son reached maturity?

If the trustee is negligent in safeguarding the money, exposing it to theft, then he is liable for its guarantee. Otherwise, he is not liable, because he is an honest custodian who is not responsible except in cases of transgression or negligence. Negligence is determined by custom, as Ibn Qudamah said: "There is no guarantee upon a trustee unless he transgresses. The summary of this is that a deposit is a trust, so if it perishes without negligence from the trustee, he is not liable for its guarantee, whether or not some of the trustee's own money was lost along with it."