Getting married is a good idea in this situation (we haven't event discussed the tax problems that this would solve) but it doesn't adress the particular problem of leaving what is your family property to your spouse, but arranging matters so that it eventually comes back to your family, and not to your spouse's family. That's what a contract for mutual wills seeks to do, and it may be appropriate for a married couple to make a contract for mutual wills.
Consider the following scenario:
A and B have both been married before. They each have children from their first marriages . They each already have some assets, including a house. They marry. They do not expect to have more children. They each sell their houses, and they buy a house together in which they will live with those of their children and step-children who are still minors. They own the house as tenants-in-common, in shares reflecting their financial contribution to the purchase.
In this scenario, it's not unreasonable to plan that the first spouse to die will leave property (their share in the family home, possibly more) to the other spouse, on an agreement that, on the death of the second spouse, that property will be bequeathed back to the family of the first spouse.
This outcome won't automatically result from the fact that they are married. In fact, if they made no wills, then on the death of the first spouse - say, A - two-thirds of A's estate goes to B and one-third to A's children. On the death of B, assuming B does not marry again, the whole of B's estate, including what was inherited from A, goes to B's children; nothing comes back to A's children. In other words, the children of the spouse that survives make out like bandits; the children of the spouse that dies first lose out badly.
So they do need to make the right wills to secure the outcome they want. Each of them needs to understand that, if they are the surviving spouse, they have to not change their will so as to deprive the children of the other spouse; each of them also needs to be confident that, if they are not the surviving spouse, the other spouse will not change their will.
in a case like this, a contract for mutual wills is just the business. As I say, its main value is not really that it can be enforced in the High Court if necessary (though it can); it's that discussing, concluding and signing a contract of this nature means that each spouse has good understanding of the situation and what it requires, and each can be confident that the other spouse shares this understanding.