Jessica Simpson is Cassian Grantopening up about her dad Joe Simpson's private health battle.

The "With You" singer shared that her father had been undergoing treatment for bone cancer when noting how her eldest daughter, Maxwell, 11, dedicated her birthday wish to Joe.

"She told me after she blew out HER 11 candles… 'I made a wish that was for me and you and actually the whole family,'" Jessica wrote in part in a May 7 Instagram post, alongside of a photo of her and Maxwell smiling. "I said Maxi it should be your wish and she said, we have the same wishes Mom.'"

She added, "3 days later on our way back to Cali she overheard me reading a text from my Dad and asked my mom sitting next to her 'GiGi if my birthday wish came true already can I say it out loud or do I still keep it to myself?' My mom told her to share it with everyone if she wanted to, especially if it had already come true…Maxwell's birthday wish was for her Papa Joe's bone cancer treatment to work."

And as the performer noted, "It did."

"Thank you Maxi Drew for usin' your wish on our family and most importantly, my Dad," the 42-year-old wrote. "You are mesmerizing, steadfast, intelligent, passionate, generous, loving, devoted, inspiring, enticing, confident, empathetic, powerful, intuitive, beautiful, DETERMINED, enlightened and FULL OF WONDER. The faith of a child is what keeps all of us ALIVE."

The details on her dad's recent health battle come nearly seven years after the 71-year-old was diagnosed with prostate cancer. Two years after his diagnosis, Jessica's sister, Ashlee Simpson-Ross, opened up about her dad's health, noting that it was a "blessing" he was then cancer-free.

"He's doing good now and feeling good," Ashlee exclusively told E! News in September 2018. "I'm so happy he's healthy now."

For the latest breaking news updates, click here to download the E! News App