Bobby or Ted Kennedy and Sargent Shriver? Shriver was a Kennedy by marriage.
Schwarzenegger with a Kennedy VP, since Schwarzenegger was married to Sargent Shriver's daughter (if you get around the birthright citizenship issue).
Hillary Clinton/Anthony Weiner if you accept the 'Huma was basically Hillary's Second daughter' argument.
The Udall Family seems like another possibility. If you include the Hunt and Lee branches of the family, they've managed to get elected in Arizona, California, Colorado, New Mexico, Oregon, and Utah and from both parties. Mike Lee of Utah (R) and Gordon Smith of Oregon (R) both are Udall family members from different states, and folks might not notice the connection. If Gordon Smith hadn't lost in 08 (a pretty close thing) then by 2011 there'd have been four Udalls in the Senate: Tom Udall (NM), Mark Udall (CO), Mike Lee (UT), and Gordon Smith (OR).