Isaac was married to Josephine Anwhistle, and the two lived in a house on a cliff overlooking Lake Lachrymose. Both were attacked, on two separate occasions, by Lachrymose Leeches, but while Ike is known to be dead, Josephine is only presumed to have died.
While Isaac and Josephine were known V.F.D. Volunteers, Gregor's position within the organization seems questionable.
Ivan Anwhistle also shared the Anwhistle surname. He once gave an S-shaped Lachrymose Leech paperweight to Lemony Snicket. His relation to the brothers is unknown, but presumably he was either their father or grandfather.
It is possible that Gregor and Ike are on the Snicket family tree; their mother D. Snicket, Josephine's mother-in-law, had only one eyebrow and one ear. Given that G and I on the Snicket family tree are Gregor and Ike, the H on the tree is a sibling of theirs; the gender, whereabouts and status of this sibling is unknown.