Q: who has the rights to a house?

A: Short Answer: If the son of her deceased husband was never actually adopted by your mother-in-law, then there is a chance that such un-adopted stepchild will not participate in her estate as one of her own children.

Long Answer: If your mother-in-law is the sole owner of the house and she dies intestate, then her intestate estate would likely be passed to her heirs as determined by the progression of heirs outlined in Colorado Revised Statutes (CRS) Section 15-11-101 and following. As you work through that statute, you'll note that if she dies without a spouse and without a designated beneficiary (pursuant to a recorded designated beneficiary agreement), then the next to take her estate under Section 15-11-103 are her descendants (per capita at each generation).

Under CRS Section 15-10-201(11), a “descendant” requires a Parent-Child relationship at each generation. CRS Section 15-10-201(7) defines “child” to specifically exclude a “stepchild” or “foster child.” CRS Section 15-10-201(36) defines “parent” to specifically exclude a “stepparent” or “foster parent.” Therefore, a “stepchild” who was not actually adopted by your mother-in-law is not considered one of her descendants and, thus, not one of her heirs in the event she dies without a will or trust in place to name that stepchild as an heir.

Given the few facts in your question, this may mean all of your mother-in-law's children inheriting equally from her estate.

This is one reason why estate planning is so important, especially in blended families. Her now deceased husband may have assumed (or hoped) that his son would still eventually benefit from his co-ownership of the house. But, without proper estate planning, the house may have passed completely into the name of his wife (your mother-in-law), who is free to dispose of it however she wants, even if that means intentionally (with an estate plan of her own) or unintentionally (by dying intestate) dis-inheriting the son. There are estate-planning techniques that might have worked if both spouses had developed an estate plan while they were still alive that was intentional about protecting all of their blended family from this result (it could have gone the other way if your mother-in-law had died first and the son had inherited everything from his dad to the exclusion of your mother-in-law's daughters). Again, this assumes the children were not adopted by their stepparents and that there are no estate planning documents that will direct the disposition of the house.