SWIMMING: Dana and Tara Kirk, United States

Stanford swimmer Dana Kirk has a way of bringing out the silly side in her usually reserved older sister Tara.

The sisters are one of three pairs of siblings who are members of the U.S. Olympic swim team. (The others are Hayley and Aaron Peirsol and Klete and Kalyn Keller.) Tara, 22, competes today in the final of the 100-meter breaststroke, where she hopes to challenge world-record holder Leisel Jones and Brooke Hanson of Australia. Dana, 20, is entered in the 200 butterfly, which begins Tuesday with qualifying .

The Kirk sisters’ rivalry has always colored their lives.

When they were children, Tara put masking tape down in the middle of the bedroom they shared. She said, “That’s your side,” and kicked her sister’s possessions across it.

Tara, who was nicknamed “Captain Kirk” by Stanford teammates, recently completed a remarkable collegiate career, winning 35 consecutive races and four NCAA titles in a row. She was named college female athlete of the year this summer.

After the Athens Games, Dana will return for her junior year at Stanford. Her mother recently told the Seattle Times that Dana initially had trepidation in following Tara to Palo Alto.

“She said, ‘I’m going to get awfully mad if the call me Lieutenant Kirk.’ “