Lyndon Sheehan and his sister, Amy Sheehan, will both compete at Sochi in freestyle halfpipe, but not for the same nation. Lyndon will represent New Zealand, while Amy skis for Australia. Apparently Olympic News Service (ONS) just announced that this is not rare, and happened twice before at the 2010 Winter Olympics, with brothers Jan (NOR) and Tommy Schmid (SUI) in the nordic combined, and siblings Anastasia Kuzmina (SVK) and her brother, Anton Shipulin (RUS), in the biathlon. ONS missed another one from Vancouver, as the Reed Siblings, Allison (GEO), Cathy (JPN) and Chris (JPN) all participated in figure skating.
But it actually first happened back in 1960 at Squaw Valley. There František Tikal played ice hockey for Czechoslovakia, while his brother, Steve Tikal, played for Australia, and they played against each other.
Here are the siblings who have competed at the same Winter Olympics, in the same sport, but for different nations:
1960: František (TCH) / Steve (AUS) Tikal in ice hockey
1998: Hans (AUT) / Bernard (SLO) Knauß in alpine skiing
2002: Robert (CZE) / Martin (GER) Reichel in ice hockey
2010: Jan (NOR) / Tommy (SUI) Schmid in nordic combined
2010: Allison (GEO) / Cathy and Chris (JPN) Reed in figure skating
2010: Anastasia Kuzmina (SVK) / Anton Shipulin (RUS) in biathlon
But it has also happened a number of times at the Summer Olympics, as follows:
1964/1968: Fred (NED) / Tony (USA) van Dorp in water polo
1984: Carmen Ionescu (CAN) / Florenta Tacu (ROU) in athletics
1992: Katerina, Maggy (BUL) / Manuela Maleeva (SUI) in tennis
1996: Gusman (MDA) / Elmadi (KAZ) Zhabrailov in wrestling
2000: Jenny (ITA) / Jessica (NED) Gal in judo
2008: Ágnes (HUN) / Erszébet (ITA) Valkai in water polo
2008: Hilda (NED) / Sylvia (KEN) Kibet in athletics
2008: Matty (USA) / Shane (NZL) Reed in triathlon
2008: Natasa (HUN) / Stjepan (CRO) Janic in canoeing
2008/2012: Soslan (UZB) / Taymuraz (KAZ) Tigiyev in wrestling
2012: Mimi (BRN) / Almensh (BEL) Belete in athletics
In addition to the Tikals competing directly against each other, this also happened with the Van Dorps in water polo in 1964; the Reichels in ice hockey in 2002, and the Valkais in water polo in 2008. In addition, the Zhabrailovs actually had a wrestling match against each other.
UPDATE (9 July 2016): As the 2016 Olympics are dawning, the brother / sister tandem of Bernard Legat and Violah Cheptoo Legat will also compete for different nations in Rio. Violah has made the Kenyan team in the women’s 1,500 metres. Bernard made his fifth Olympic team at the US Olympic Trials in the 5,000 metres. He competed for Kenya in 2000 and 2004 and for the United States in 2008 and 2012, and to come, in 2016..