Legal Question in Real Estate Law in Massachusetts

Not likely. Absent proof that the items were stolen by the person who ultimately supplied them to the store and/or that you are the heir entitled to them you cannot assert the right to them. Unless these things are worth a great deal to you emotionally or are very valuable the cost of asserting those rights will be exorbitant.

You could ask the store how they came by the photos and the letters. Depending upon the answer, it will give you an idea of what rights you might be able to assert. You need to be absolutely positive you are family. If there are other family members you will need to deal with their rights as well.