When did phoebe fraunces die?

Samuel Fraunces's eldest daughter, Elizabeth, was born December 26, 1765. There is a theory that her nickname was "Phoebe", but no evidence to support this. Elizabeth would have been a 10-year-old at the time of the supposed June 1776 assassination plot against George Washington. If a "Phoebe" existed, she may have been a woman enslaved or employed by Samuel Fraunces, rather than his daughter.