Known as Justina to her friends and relations, Brandly Lazarus, only child of Frumet Cohen and Sampson Lazarus, was likely born in Fredericktown, Maryland, where her father, uncle, and both grandfathers were well established in trade. However, the family had limited communal support in such an isolated spot. Brandly was 27 and still unmarried when her uncle Isaac, who had been her father’s business partner, died childless in April 1779. A year later, Sampson Lazarus moved his family to Lancaster, Pennsylvania, hoping to improve his daughter’s prospects for marriage. The strategy paid off, and in 1781 Brandly was married to 36-year-old Joshua Isaacs, Jr., then a Pennsylvania militia man serving the cause of American independence. Their first two children, Joseph and Frances, were born in Lancaster, however shortly after the end of the war, the young couple moved to New York, where their four younger children—Solomon, Grace, Sampson, and Hannah—were born.

Brandly seems to have been adored by her children: Frances named one of her children for her, and son Solomon named two. Brandly, who would survive her husband by 15 years, died on the anniversary of his death, and was buried beside by him in the Chatham Square Cemetery in New York City.