Britney and Jamie Lynn Spears showed off their bikini bodies in Miami Monday afternoon.“They both looked to be in pretty good shape,” a witness tells Usmagazine.com of the sisters, who spent about an hour at the Mandarin Oriental Hotel pool Monday afternoon with their kids and parents.It’s the first time 18-year-old Jamie Lynn, who welcomed daughter Maddie in June 2008, has shown off her post-baby body. “She had on a white, spangly top,” the witness says. “A very little bikini!”