Will the woman who received jewelry from her deceased aunt be sinful for taking it without the knowledge of the heirs, given that the aunt had gifted it to her during her lifetime, and is she obligated to return its value to them?

It is not permissible for you to take the jewelry unless it does not exceed one-third of your aunt's estate. If it exceeds one-third, then it is not permissible for you to take it except with the permission of all her heirs. This is because a gift made after death is considered a bequest, and a bequest to a non-heir is permissible, but it cannot be enforced for an amount exceeding one-third except with the heirs' permission. This is based on the Prophet's (peace be upon him) statement to Sa'd ibn Abi Waqqas: "One-third, and one-third is a lot. Indeed, that you leave your heirs wealthy is better than that you leave them dependent, begging from people."