The married life has certainly been good to Jessica Simpson and her husband, Eric Johnson!

The couple went on a family vacay over the weekend to Cabo San Lucas, Mexico -- where they met up with Jessica's sister, Ashlee Simpson, Ashlee's husband, Evan Ross, and the girls' mother, Tina Simpson -- and showed off their enviable physiques. The 35-year-old fashion entrepreneur flaunted her super flat stomach in a sexy two-piece bikini, while the former NFL pro displayed his six-pack in nothing but a pair of swim trunks.

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The vacation looked to be a relaxing and fun getaway, with Eric playing beer pong with friends and doing cannonballs into the pool, while Evan served everyone drinks, an eyewitness tells ET. Ashlee and Evan's adorable daughter, the almost 6-month-old Jagger Snow, was also present.

The Simpson family was likely celebrating Tina's birthday over the holiday weekend. The mother of two turned 56 on Monday.

... And we know Tina is certainly fond of both of her son-in-laws. On Monday, she hilariously Instagrammed a pic of Eric, 36, doing a pose between her legs. "Son in law," she captioned the shot.

Tina also held hands with 27-year-old Evan poolside, writing, "Blessed by the 2 best son in laws!!!"

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ET caught up with Jessica earlier this month at the YMA Fashion Scholarship Fund Awards Gala in New York City, where she dished on her successful diet and exercise plan.

"I think being a mom is enough of a diet," Simpson, a mother to her adorable kids, Maxwell Drew and Ace Knute, said. "You have to run around chasing your kids, but I do eat a lot of their corn dogs, so I have to get on the treadmill a lot to watch my TV shows."

Well, whatever it is she's doing, it's clearly working for her!

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