The eldest of twin daughters born to Isolt Sayre and James Steward, Martha was named for her paternal grandmother. During the attack on Ilvermorny when she and her sister Rioach were babies, it was their frightened screams that broke the curse upon their parents and thwarted Gormlaith Gaunt’s plans.
As a Squib, she found herself unable to bear living at Ilvermorny while unable to produce magic herself. She left the castle and married a man from the Pocomtuc tribe, living the rest of her life as a No-Maj (Pm).
Interestingly, Pottermore says that Martha was loved by her parents and the Boot brothers, but her twin sister Rionach is not mentioned. It is possible that discord with her twin factored into Martha's sense of not belonging at Ilvermorny. Perhaps it reflects the troubled relationship between their grandmother, Rionach Sayre, and her sister, Gormlaith Gaunt. -BB