Jon and Kate Gosselin Celebrate Twins' 18th Birthdays With New Messages

The eldest Jon and Kate Plus 8 siblings are officially adults.

On Monday, Cara and Mady Gosselin, the oldest of the Gosselin eight, celebrated their milestone 18th birthday and their parents, Jon and Kate Gosselin, took to Instagram to help them ring in the special day.

"Happy 18th birthday!!! To my twin girls, Mady & Cara!!! Love, Dad," Jon wrote in a photo shared with his Instagram followers.

The twins' mother, Kate, wrote a lengthier note, sharing several photos from the birthday preparations, which included a birthday cake and customized M&M candies.

"Getting ready.........it's a BIG busy day here! More to come.... HAPPY 18th Birthday, Cara and Madelyn!" Kate wrote. "There are no words to describe how proud I am of you both. Despite the many turbulent times you've had to deal with so far in life, you have emerged as wise, level headed, reasonable, forgiving, loving, kind and absolutely brilliant ADULTS!"

"I can say that I've poured endless love into you and fought tooth and nail for the best for you, but I cannot possibly take credit for the truly wonderful humans (adult humans!) you have become! You will thrive in life, I am certain, and I will still be with you every step of the way going forward, continuing to guide you and love you," she continued.

"I love you both so very very much and I wish peace, happiness and the great successes I know are coming your way!" Kate concluded, adding the hashtags "#Turning18 #Adults #Love #Happy18thBirthdayCara #Happy18thBirthdayMady."

The twins' birthdays come just weeks after their sextuplet siblings — Hannah, Leah, Alexis, Joel, Aaden, and Collin — celebrated their 14th birthdays in May, though both birthday celebrations have been overshadowed by a fierce custody battle.

Although Kate reportedly maintains full custody of all eight children, it was also reported that Hannah had been living under her father's roof full-time while her parents were "still at odds." Meanwhile, the twins had become completely estranged from Jon, telling PEOPLE in August of 2016 that they no longer spoke to their father.

"He makes it seem like we're being kept from him, which is insane," Mady said. "He should maybe spend some time thinking about why we don't want to see him, and maybe realize that if he ever does want a relationship with us, talking about us on TV is not the way to make that happen."

"He doesn't even know us," she added. "How can he dare to talk about us?"

The Gosselin family first rose to fame in 2007 when their TLC series Jon and Kate Plus 8 premiered. Following their split in 2008, the series went on to become Kate Plus 8 until it went off air in 2017. Kate is expected to return to the network in a new series documenting her search for love, Kate Plus Date.