There are only two ways to determine what "rightfully" belongs to your husband: his mother's will, or if she died without a will, the intestacy laws in the state where his mother resided at the time of her death. In Massachusetts, for example, if someone dies without a will, the estate is automatically divided by law as follows: 50% to the surviving spouse and 50% divided between the surviving children. (Obviously, determining what assets are owned by the estate and how they should be divided can sometimes present some challenges, but it must be done.)
Massachusetts law provides that “[a] person having custody of a will, other than a register of probate, shall, within thirty days after notice of the death of the testator, deliver such will into the probate court having jurisdiction of the probate thereof ..." M.G.L.A. 191 § 1. In other words, if you have a true copy of Massachusetts resident's will in your possession, you must file the will at the Probate and Family Court within thirty days of learning that the person died. Not everyone knows about this law, but is a good thing to reference if someone claims to have a copy of a deceased person's will but is dragging their feet on producing it. Once filed at court, anyone can obtain copies.
If the mother had a will, then an executor will be appointed by the court (executor chosen based on the individual named in the will unless there is some clear reason the named individual should not be executor). If she did not have a will, a family member would petition the court to be appointed administrator of the estate. In either event, the executor/administrator would be obligated to survey and account for all property owned by the mother at the time of her death, and to then divide the estate by the terms set forth in the will or the laws of intestacy. It is important to note that the individual appointed executor/administrator will "stand in the shoes" of the mother, and be able to transfer and/or sell assets as if they were the mother. (In doing so, however, they will be bound by their fiduciary duty to the estate, and will likely be found personally liable if they act contrary to the best interest of the estate - particularly if they have enriched themselves at the estate's expense.)
While I would need to know a lot more to say anything definitive, it sounds like two things are in order: determine if a will exists and seek the appointment of an executor/administrator.
Please note that the information set forth above is not legal advice, and should not be construed as such. Rather, it is my response to the fairly simple factual scenario up present, which I would need to review and verify in detail before rendering any legal advice. My law firm does this type of work and I'd be happy to provide a free one hour consultation to your husband. There are a variety of ways to approach legal probate issues of this nature.
First of all I am sorry for your loss.
It sounds like your mother in law had accumulated a number of assets during her lifetime but do you know how the assets are held? If the real estate was held as a joint tenancy with her husband then the real estate automatically goes to him. He could potentially change his Will or deed the property in a manner that causes assets to go to his children and there is little you can do to prevent this, however, you can stay informed as to how the real estate is held by periodically checking title.
It is impossible to answer your question without additional information. Specifically, we do not know what assets "rightfully belong" to your husband unless we know what is provided in his mom's Will, among other things.
After your husband has had time to come to grips with his loss ask around to see if there is a Will. Then check title to the real estate. Only then will you begin to see what assets go where.
Your husband should arrange to sit down soon with an attorney who deals with Probate law in the state where his mother was domiciled at the time of her death. He may later need to also contact an attorney in that state "up north" where the farmhouse is located.
First he needs to gather information on what assets were owned and how they were owned. As pointed out by one of the earlier answers, your husband may be entitled to a share of his mother's probate assets as her next of kin (heir). He may also be entitled to a share of the estate under his mother's will, if she left a will. There may also have been a trust under which your husband may or may not have been a beneficiary. Finally, there may be other nonprobate assets with your husband as the beneficiary such as an IRA account, a 401K, life insurance policies, annuities, and many other possible assets.
For the most part any property rights of your husband will be only those his mothers took steps to protect for him, but he can lose even those if he is not diligent.