According to Official Guinness Records,
The most siblings to complete a marathon is 16 and was set by the Kapral siblings: Chris, Vince, Steve, Tony, Joe, Doug, Angela, Allison, Theresa, Phil, Michelle, Mary, Nick, Mike, David and Sarah (all USA) who ran the Community First Fox City Marathon, Appleton, Wisconsin, USA, on 20 September 2009. The brothers and sisters, ranging in age from 18 to 44, posted the times from 3:09:51 to 5:31:49 in the 26.2 mile (42.2 km) race.
For a complete list of 2009 records, please visit 2009 Guinness Records in United States.