"Him" referring to the FFL not my deceased father-in-law.
Well, that makes more sense.
I thought I had read somewhere on this board that FA-10's were necessary for firearm transfers on guns coming into the state as well as those leaving the state. I do have to admit that I have been searching for about a couple of hours for that post and I cannot find it again.
IF so, you read incorrect info.
Your wife is, according to your post, the executrix. She has not only the right, but the duty, to allocate the assets of the estate per the terms of the will.
Since it is not specifically addressed, the gun would be part of the residuary clause of the will; chattel to be apportioned among the heirs. In this case, it appears there are only two; your wife and her brother.
She could buy his half of the gun from her brother, or he could trade his share to her for something else from the estate; the gun could be sold and the proceeds divided. So long as all property is duly accounted for and all beneficiaries sign off on the assent, the estate can be probated and closed.
As executrix, she can take possession of the gun with the other property. The problem of licensing and transport may be addressed through your happy possession of an NR LTC.
Assuming the brother is on board with
your getting
his father's gun, AND your wife has obtained her appointment as executrix from the Probate Court, the procedure is for the gun to be transported per FOPA from MA to your home state, and to have an FFL transfer it from your wife as executrix to you.