But are his daughters happy about the May/December match of the 68-year-old composer and Scorpion star, 34?
Foster's Erin, who is just a year older than McPhee, commented, "Mommmyyy" along with three red heart emojis.
Her sister, Sara, jokingly wrote, "Out of the country. What did I miss?"
Former American Idol runner up McPhee, who was mentored on the reality show by Foster way back in 2006, is now marrying him despite a more than 30-year age difference.
McPhee, an actress as well as a singer, posted a photo on Instagram with model friend Mayra Suarez on a yacht with her new left-hand engagement ring sparkling in the sun.
She and Foster got engaged while on a romantic trip in Capri, Italy, as the famous composer confirmed on his Instagram with the caption, "Yup!"
McPhee shared on Instagram about Foster's proposal, "He did it at the top of this mountain in Anacapri. Totally dark, only stars."
It will be the fifth marriage for Foster, and the second for McPhee, who divorced actor Nik Cokas in 2016.
The Foster daughters apparently had a tense relationship with their dad's previous wife, c Yolanda Foster.
As Radar reported, they accused her of faking Lyme disease for reality show ratings!
Sara told an interviewer about Yolanda's drama at the time, "Only she knows her journey, but I will say is if you're capable of continuing to be on a reality show, you're not dying."
Foster's romance with McPhee seems to be hot and happy despite the age gap, as their big PDA moments have shown.
And despite their sarcasm, his daughters apparently approve of young Kat!
In October, when the two were dating, Erin told PEOPLE, "She's amazing. I really like her. She's awesome. My dad is kind of a hopeless romantic, and Katharine's awesome. We really, really like her, and we've known her for a really long time. And if he's happy, we're happy."
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