Their deal with owner Stephen Ross, first reported last week, made the sisters the latest celebrities to become minor owners in the NFL team.
Venus and Serena live in Palm Beach Gardens, about an hour's drive from the Dolphins' stadium.
"To have this opportunity is really where our heart is," Venus said. "We're South Florida girls. When we get off the road, this is where we come home to. When we come home to Dolphins games, it's going to be exciting."
The sisters are the first female African-Americans to hold an ownership stake in an NFL team, the Dolphins said. The NFL has no African-American majority team owner.
"We're really honored," Serena said. "Venus and I in tennis have tried to do so much for the sport. We're really excited to even have this opportunity."
Venus said she and Serena have been to "a few games." Serena dated former NFL players LaVar Arrington and Keyshawn Johnson, but the sisters said the Dolphins have long been their favorite team.
"We're just 'Go Fins!'-type people," Serena said.
"It's great going to the games for us," Venus said. "When we play tennis, you're so focused you don't really get to feel that atmosphere. When we go to a Dolphins game, we get to soak in the atmosphere and we realize, 'Oh my God, we do this too."'
The sisters posed for photographers holding team jerseys - No. 11 for Venus, No. 89 for Serena. EleVen is Venus' clothing line; 8-9 is the birthdate of their half sister, Yetunde Price, who died in a 2003 shooting.
"We are thrilled to have Venus and Serena join the Dolphins as limited partners," said Ross, a New York real estate billionaire. "They are among the most admired athletes in the world and have become global ambassadors for the game of tennis. Their addition to our ownership group further reflects our commitment to connect with aggressively and embrace the great diversity that makes South Florida a multicultural gem."
Serena has won 11 major titles and Venus seven. They staged their latest sibling showdown last month at Wimbledon, where Serena beat Venus in the final, and will play in the U.S. Open beginning next week.
Musicians Gloria and Emilio Estefan and Marc Anthony recently bought small shares of the team. The Estefans are the first Cuban Americans to hold an ownership stake in an NFL team, and Anthony is from New York of Puerto Rican descent.
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