Billie Tisch, the last of the original four family members whose philanthropic legacy endures in the prominent educational, cultural and medical institutions that bear their surname, as well as in their prodigious fundraising for Jewish social service agencies that have anonymously sustained countless New Yorkers, died Sunday at her home in New York City. She was 98. Her death was confirmed by her son Thomas Tisch. Billie Tisch was the widow of Laurence A. Tisch.