My grandmother Neill had a brother named Cecil. Her sister's husband was named Cecil and her husband (my grandfather) was named Cecil. While I don't know about the brother-in-law's family, I do know that the name of Cecil had not previously appeared in either my grandmother or grandfather's family.
Apparently at the time of these births, between 1900 and 1915, the name was fairly popular. It wasn't all that popular say fifty years earlier and fifty years later, its popularity was waning.
There may be a reason a name "appears out of thin air" in one of your families. Just remember that the name may have no genealogical connection to any other family member. It just might have been in fashion.