Her parents created her, of course
Was she an only child, or one of a litter?
If we accept the bio originally sold through LE and reprinted at Danhauser's site (linked on the previous page), then M'Ress has at least three "litter-mates".
Their family units are extremely close. M'Ress's other three litter mates also serve in Starfleet in various capacities.
Alan Dean Foster actually provided names for these siblings and her parents in the Log novels. (I had jotted them down a years ago for "quick" reference, but Mogg only knows where I left that slip of paper.) She either has 1 sister and 2 brothers, or maybe 2 sisters and a single brother; I'm not sure. But I do know all three were not the same gender. Foster also implied M'ress was the eldest since her "Mater" (the term ADF used) stated they "looked up to her as something of a role model.
A little follow-up on M'ress' family. Here are the list of immediate relations, parents and siblings as described by Alana Dean Foster in the "...Log" novels.
M'Mar...mother (addressed as "Mater")
M'Nault...father (referred to as "Sire")
Three friends were mentioned, all male.
Oh, this info came from "Memory Alpha".