Ted Kennedy and Bobby Kennedy were Senators together.
What's the largest number of Kennedys who could be in the Senate at the same time?
Shrivers count as Kennedys for this purpose.
JFK losing in 1960 + Bobby getting elected in 1964 (if he still runs) + Ted staying in office gets you three brothers in the 60s.
Caroline Kennedy was interested in replacing Hillary Clinton in 2008, and Patrick Kennedy considered challenging Lincoln Chafee in 2006. That could mean that in 2008 you'd have Ted, Patrick, and Caroline Kennedy in the Senate together. This would decrease when Ted passes away, but I could see Joe Kennedy III getting elected in 2012 (instead of Warren, should he run to succeed Bill Dellahunt in 2010) or 2014 (instead of Markey).
Ted Kennedy Jr could get elected to Senate in 2012 in Connecticut perhaps.
Timothy Shriver looked at running for CA AG in 2010. Maybe he beats Harris and proceeds to be Senate nominee in 2016.
Kathleen Kennedy Townsend could get elected to Senate in Maryland in 2016. She was LtGov of the state from 2005-2009.
If you really wanna stretch it, I guess Schwarzenegger could win Senate and count as a Kennedy by marriage.