The 29-year-old actress and singer posted Instagram photos from celebrations to mark Alexandre's 18th birthday, showing him with several birthday cakes adorned by sparkler candles.
"It’s a Celebration. Joyeux anniversaire mon petit frère Alexandre!" Jazmin wrote in the caption.
WATCH: Princess Charlene and Prince Albert share highlights from their relationship
Albert, 63, confirmed he was the biological father of Alexandre whose mother is Nicole Coste, a former Air France flight attendant from Togo, in May 2005, just before he was enthroned as Prince of Monaco.
Alexandre was born on 24 August 2003 in Paris, France.
Jazmin shared photos from Alexandre's 18th birthday celebrations
The royal also fathered Jazmin, born in 1992, with real estate agent Tamara Rotolo. She was confirmed as Prince Albert's daughter in 2006 when she was almost 16 years old. He had previously said he wanted to protect her identity until she was an adult.
Neither Jazmin nor Alexandre are in the line of succession to the Monegasque throne as their parents never married.
Jazmin pictured with her father Prince Albert in February 2020
Albert married South African Olympic swimmer Charlene Wittstock in July 2011 and the couple welcomed twins, Prince Jacques and Princess Gabriella, in December 2014. Jacques is the heir apparent to the throne.
Princess Charlene, 43, shared some photos of her reunion with her husband and their children in South Africa last week after recovering from a four-hour operation.
She has remained in the country since May after she came down with an ear-nose-throat (ENT) infection. She has been unable to return to Monaco since then and missed her tenth wedding anniversary with Prince Albert in July.
The snaps shared to her Instagram account showed her cuddling up to her husband, while others showed Prince Jacques and Princess Gabriella beaming as they climbed trees.
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