Is the son's wife sinful for selling the gold ring found by her mother-in-law, without informing the mother-in-law of its value, knowing that the ring was found a year and a half ago and no one reported it, and informing the mother-in-law now may cause problems?

The daughter-in-law sinned by concealing the ring entrusted to her. She neither returned it to its owner nor made known the finding; instead, she spent it without declaring it. She must repent to Allah and give charity equal to the value of the ring in the name of its owner. If the owner is found later, she is obligated to guarantee its value to her.