The Manchester by the Sea star sent his best wishes to the happy couple, who reconciled last year after calling off their 2003 nuptials, on Sunday, August 21, following the pair's barbecue and picnic for their attendees.
Sharing a throwback photo of himself walking beside Bennifer from the 2000s, Casey captioned his Instagram post for the newlyweds: "Good things are worth waiting for. Here is to twists and turns, new beginnings, and finding new reservoirs of old love."
"Welcome to the family," he penned. "Get ready for some real dysfunction! Kidding. I am kidding."
Casey concluded, "Jen, you are a gem. We love you so much!!!"
Ben, 50, and Jen, 53, kicked off their wedding festivities Friday, August 19, with a rehearsal dinner at the Argo actor's Georgia estate, where the Hollywood power couple's nuptials took place.
Casey didn't direct acknowledge why he was skipping his older brother's special day, but when asked about it by a celebrity photographer in Los Angeles on the morning of the Saturday, August 20, event, the Gone Baby Gone actor muttered something that sounded like, "I have other things."
Though he didn't elaborate further on why he couldn't be there for his brother, a source told People he had "family, parental obligations at home." Casey shares sons Indiana, 18, and 14-year-old Atticus with ex-wife Summer Phoenix.
Meanwhile, his girlfriend, Caylee Cowan, accidentally gave away his real weekend plans, reportedly spilling that he was on dad duties.
“Hey, if you weren’t in town Saturday to take Atty to soccer practice then who would’ve helped the kid who broke his arm?” she commented under Casey's Instagram post, per Page Six. “You’re a good man. I love you.” Caylee has since deleted the comment.
Aside from the Affleck family not all being in attendance, the couple's former partners also opted out of the star-studded event. Ben's ex-wife, Jennifer Garner, unfortunately had a work obligation that kept her away from the three-day affair, while Jen's ex-husband, Marc Anthony, was seen in Miami.
And while Casey insisted he has nothing but love for his new sister-in-law, OK! previously reported the famous sibling isn't exactly a fan of Ben and Jen's highly-publicized relationship.
“He finds them superficial,” spilled an OK! insider. “Ben and Jen are at the center of the whole L.A. social scene, which Casey finds frivolous and has really divorced himself from.”
“Casey is allergic to the circus Ben’s life has become, and it’s just not a world he knows or cares to navigate. He and Caylee won’t be double-dating with Bennifer anytime soon!" concluded the insider.