The children of John Houston (15) are variously identified by genealogists. The core list of children is commonly given as
This lists corresponds to that provided by the Rev. Samuel Rutherford Houston in Source:Houston, 1882. Given that the Rev. Houston was John (15)'s grandson, we can probably accept his list as highly likely. Some of these children can be independently confirmed. John (15)'s will, for example, identifies only two children, sons Samuel and Mathew. They may have been called out for specific mention since they seem to have been underage at the time their father wrote his will in 1748.
Independent verification of James, Robert, isabella, Esther, and John is needed, but we can accept them "pro temp".
A child sometimes included in this list is Mary Houston. The basis for her inclusion is uncertain. However, a Mary Blair is explicitly mentioned in Johns will as someone who should have a share with the rest of the children. His will states
that Mary Blair have a share with the rest; and likewise,
Most researchers that mention her (per Ancestry World Trees) identify her as a daughter of John, whose married name was "Blair". At this point we do not have enough information to explain why John felt the need to mention her specifically, or why, if she is John's daughter, she bore a different surname. It would seem unlikely that "Blair" was simply her married name, as there would have been no obvious reason for stipulating that she should be treated no differently than "the rest".
There are several possible interpretations of this:
Any of the above could explain her presence in John's will. Possibly there is yet another explantion. One researcher notes that "Matthew Lyle and Esther Blair were the parents of Martha Lyle who married Matthew Houston. (Possibly Mary Blair was a relative of Esther Blair Lyle?)".. Another researcher notes, without any reference to the Houston's, that
Mary was born in 1726 in Ballyvoy, Antrim, Ireland. Mary's father is Robert Blair and her mother was Martha Campbell Lyle. She was an only child. She died at the age of 95 on August 12th, 1821 in Timber Ridge, Rockbridge, Virginia. Goodwin Family, April 2012
If correct, the above suggests that her inclusion in John's will was indeed based on some degree of family relationship, but not that of father-daughter. Additional work on Mary Blair is needed. Additional information on her supposed parents Robert Blair and Martha Campbell Lyle is needed.
While "James" is identified by Rev. Samuel Houston as a son of John, Houston tells us that he died in Ireland, and never immigrated. Nonetheless, some authors identify a person:James Houston (17) as John's son, and to have lived on Beveley's Manor. Why he is identified by some as the son of John (15) is unknown. Additional work needed, but it seems likely that he is not a son of John (15).