My grandfather was raised by what was then called a bachelor farmer and his maiden sister. Large families were the norm but this 2 year old’s mother had died giving birth to the youngest of, I think, 10 children. The rest were fostered out to the oldest children and aunts and uncles but the father, my great grandfather, moved from West Virginia to Missouri to live with his best friend from the Civil War, George Sheppard. But in big families there were often siblings who did not get married. Often they had helped raise the younger siblings and weren’t interested in having a big family. Sometimes they took care of the parents or an older sibling. This same family - my grandmother was cared for by her younger sister, the maiden aunt.
I read somewhere that Mary was a consecrated virgin who had been raised largely at the Temple where she helped create and repair the many robes and curtains required in the rituals.
Many women in the old days if they were not married by a certain age they probably never would be. Then there were the ones who may have married but were widowed, perhaps before having children and stayed single there after.
Also there were laws about marrying first cousins and such there might not be people around to marry who were not related. I have heard this can be a problem in the Huterites and Amish and Mennonite communities even today.