That would be Richard, Bob's twin. Richard did join all his siblings in 2000 for their first (and only shows with all his siblings) at Taste of Rhode Island 2000.
John Cowsill's birth name is John Patrick Cowsill.
Paul Cowsill's birth name is Paul Mitchell Cowsill.
Susan Cowsill's birth name is Susan Claire Cowsill.
Travis Cowsill's birth name is Travis Taylor Cowsill.
Richard Cowsill was born in 1950.
Bud Cowsill was born on December 2, 1925.
Bill Cowsill went by Billy.
Bill Cowsill was born on January 9, 1948.
Barry Cowsill was born on September 14, 1954.
Barry Cowsill was born on September 14, 1954.
Susan Cowsill was born on May 20, 1959.
Tiffany Cowsill was born on September 5, 1974.