George Pimentel/Getty Tom Brady and Gisele Bündchen
"Happy birthday to a true angel on earth! I love you so much sis!" the message, translated from Portuguese, said.
The photo shows Bündchen's sister Graziela in blue, giving the Brazilian supermodel, 42, a hug as Gisele pulls a funny face with a leg kicked up. Gisele used Instagram stickers to make it look like she is holding a birthday cake and added animated hearts to the photo.
The birthday post comes as sources tell PEOPLE that Bündchen and Brady have "hit a rough patch" in their marriage.
Brady and Gisele — who share son Benjamin Rein, 12, and daughter Vivian Lake, 9, along with the quarterback's son John "Jack" Edward, 14, whom he shares with ex Bridget Moynahan — have been married for 13 years after meeting through friends. The couple tied the knot one month after they got engaged in an intimate ceremony on Feb. 26, 2009, at St. Monica Catholic Church in California.
Sources have revealed to PEOPLE that the couple is in disagreement about the Tampa Bay Buccaneer's quarterback decision to play another season in the NFL — a choice that has led to "a lot of tension" between the longtime couple.
Over Labor Day Weekend, Brady and Gisele appeared to spend time apart. Sources told PEOPLE that the supermodel took Benjamin and Vivian to a Florida water park, while Brady flew to New York to see his son Jack's football practice.
Gisele "wasn't thrilled" with her husband's decision to end his retirement from professional football earlier this year after only two months, a source close to Brady told PEOPLE.
"They're hitting a rough patch," the insider continued. "But I know they're trying, or at least he's trying, to figure it out and make it work. He wants to get through this and for things to get better."
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A source with the Buccaneers told PEOPLE that there are no preparations being made for Gisele to attend her husband's first game of the season, which happens Sunday.
"It's always a big thing when Gisele is coming to watch the game, [but] as of right now, there's none of that for Sunday's game," the source said.
"It feels very different this year than last year. She was excited to be here, we saw her around," the source continued. "I'm not saying she can't possibly come, but nothing is in place for her to be here for the first regular season game and that seems weird."
RELATED VIDEO: Tom Brady and Wife Gisele Bündchen 'Hitting a Rough Patch' in Marriage: 'There's a Lot of Tension'
The insider added that people are not bringing up Gisele around Brady.
"Everyone here knows that there's tension, but we don't know how serious it is. So we're just not talking to him about Gisele. It's not a topic anyone wants to bring up to him."
Rumors of a rift between the couple increased after Brady was excused from practicing with the Buccaneers last month "to deal with some personal things," head coach Todd Bowles said during a press conference at the time. According to Bowles, Brady and the team discussed the absence before the start of training camp in July.
Brady spoke about his absence from the team after he eventually returned 11 days later.
"Everyone has different situations they're dealing with, and we all have unique challenges to our lives," Brady told reporters. "I'm 45 years old, man."
"There's a lot of s— going on, so you just have to try and figure out life the best you can. You know, it's a continuous process," he continued.