“My brother and his wife live on the second floor of a two family home in New Jersey. My sister-in-laws brother and his wife live on the first floor. The names of all four people are on the mortgage, but only three of the names are on the deed; my brothers name is not on the deed. My brothers wife has been very ill for the last few years. If she were to pass away who would own the property?”
Although they are often the same, ownership of real estate and owing money on the mortgage do not have to be identical. So the answer to the question “who owns the property” starts with whoever is on the deed.
That having been said, anyone who is paying for the property but is not on the deed could have a legal argument that they are entitled to an interest, because it would otherwise be unfair. That’s something for the parties to work out (or if not, then sue over).
What happens when a co-owner dies depends on two things: first, how the title is held, and second, what kind of estate plan the dead person has. If the property is held as “joint tenants” or with “right of survivorship” or similar language, then the property automatically passes to the surviving owner(s). Otherwise, the dead person’s share of the property passes like all their other property: however their estate plan says, or by law if they have none.