Modesty Barebone was the youngest adopted daughter of Mary Lou Barebone, a Second Salem leader in 1920s New York.
Modesty's bedroom was spartan and dreary. Lit by an oil lamp, there was an "Alphabet of Sins" on the wall, and a shelf with a witch doll tied to a stake and another with a noose around the neck (WFT).
At eight years old, she watched her mother attacked and killed by a violent Obscurus which was triggered when Mary Lou threatened to beat Modesty with a belt for having a toy magic wand under her bed (WFT).
Auror Percival Graves believed Modesty might be a witch Obscurial who was responsible for the death of her mother, so he sought help from her brother Credence in finding her after Mary Lou's death. The frightened little girl was hiding in the run-down tenement where she once lived with her twelve siblings before being adopted. When Graves found her and sensed her fear, he suddenly realized she was not the magical source of the Dark Obscurus just before the real one burst forth and attacked him (WFT).
Family
Adopted by abusive fanatic Mary Lou Barebone along with two siblings. Because she was adopted, there was no blood connection to the earlier ancestor Bartholomew Barebone except being raised with the same anti-magic obsession.
Other canon notes and references
Modesty sang two twisted songs referring to witch-hunting while playing the child's game hopscotch on the sidewalk outside Second Salem Church:
My momma, your momma,
gonna catch a witch
My momma, your momma,
flying on a switch
My momma, your momma,
witches never cry
My momma, your momma,
witches gonna die!
Witch number one, drown in
a river!
Witch number two, gotta
noose to give her!
Witch number three, gonna
watch her burn,
Witch number four, flogging
take a turn.
Commentary
Etymology
From Latin modestia "moderation, sense of honor, correctness of conduct"
The names Chastity, Modesty, and Credence are reminiscent of Puritan naming conventions (Source: Slate).
"Barebone" hints at the Puritan Sourer legacy Bartholomew Barebone and his family carry on; first, it alludes to the austerity of Puritan sensibilities; second, Praise-God Barebone was an English Puritan of the Commonwealth/Interregnum (the period of English history between the English Civil War and the Restoration of the monarchy, when Puritans basically controlled the government and instituted reforms related to their religious and moral beliefs) who belonged to a sect particularly concerned with Judgement Day and whose name is used in reference to the appointed parliament which immediately preceded Oliver Cromwell being made Lord Protector (Barebone's parliament). (Sources: Wikipedia 1, 2, 3, 4, 5)
Notes
Unlike her more obedient sister Chastity, the younger Modesty has displays of emotion, tossing the Second Salem leaflets into the air for fun, and showing empathy for her brother Credence when he is beaten with a belt. Having a toy magic wand under her bed was certainly a risky display of rebellion towards their unforgiving mother-figure. One would hope that after the death of her mother she was able to lead a more normal and happier childhood.
From the Web
Modesty Barebone on Harry Potter Wiki.