Michael Douglas and Catherine Zeta-Jones marked another year of life and love together by celebrating their shared birthday, but one phrase from Douglas left fans doing a double take.

The longtime couple, who were both born on September 25 but 25 years apart, exchanged sweet tributes on social media. 

Douglas, now 81, posted a photo of himself and Zeta-Jones on the golf course, where they recently competed in the Celebrity Ryder Cup. 

Alongside the snap, he wrote, “To my birthday sister—what a joy celebrating another year with you! Congratulations on your spectacular game at the Celebrity Ryder Cup. Happy Birthday to the love of my life @catherinezetajones.”

While heartfelt, the term “birthday sister” raised eyebrows in the comments. 

“That’s his wife not his sister, what do he mean birthday sister?” one fan wrote, echoing the confusion of many others. 

Another chimed in, “Did you just call your wife sister?? Or am I tripping??” 

The playful expression was clearly meant to highlight their shared birthday, but it still sparked a lively reaction online.

Zeta-Jones, who turned 56, kept her own message more straightforward but just as loving. 

“Happy Birthday Michael! Sharing this special day with you is once in a lifetime!,” she wrote, making it clear how special it is to share the date with her husband.

Their daughter, Carys, 22, joined in on the family celebration with a touching tribute. 

Posting a photo of Douglas holding hands with both her and Zeta-Jones, she wrote, “September 25th, Happy Birthday Mom and Dadda!! Your love of life, love, learning, art… there are no words to describe my gratitude and admiration.”

Douglas and Zeta-Jones have been married since November 2000, and their family has Carys and 25-year-old son Dylan. 

Douglas is also father to 46-year-old Cameron, from his previous marriage to Diandra Luker.

Confusion aside, the couple’s joint birthday turned into a celebration of love, family, and a little bit of humour, proof that after more than two decades together, Douglas and Zeta-Jones still know how to keep things lighthearted.