The Hills: New Beginnings star took to Instagram on Sunday to share a reflective post seemingly hinting at Carter's recent PDA with Cyrus, writing, "Don't let yesterday take up too much of today."
On Saturday, ET obtained photos of Cyrus, 26, and Carter, 30, locking lips at a hotel in Lake Como, Italy, less than two weeks after Carter's split from Jenner. Cyrus' rep, meanwhile, confirmed that day that she and Liam Hemsworth had separated after seven months of marriage.
Jenner's Hills co-star, Brandon Thomas Lee, commented on his pic, "Let's round this scandal out and post a pic of us making out," with a kissy emoji.
"Watch out! Pics of Liam and I holding hands on the beach coming soon," Jenner replied -- clearly poking fun at the situation.
Then Cyrus chimed in, writing, "@brodyjenner go take a nap in your truck and cool off #HotGirlSummer."
A source told ET on Sunday that Cyrus and Hemsworth have spent time as a couple with Jenner and Carter. "The [ex-]couples are friends -- the four of them have spent time together in Malibu," the source shared. "Brody and Liam are both into surfing and are friends."
In a statement to ET on Saturday, Cyrus' rep said she and Hemsworth were "lovingly" taking time apart. "Liam and Miley have agreed to separate at this time," the rep said. "Ever-evolving, changing as partners and individuals, they have decided this is what’s best while they both focus on themselves and careers. They still remain dedicated parents to all of their animals they share while lovingly taking this time apart. Please respect their process and privacy."
"It's just tough, you know?" he said at Variety's Power of Young Hollywood gala. "It's tough because I'm close with both of them and they really do love each other. I mean ... they were such a great couple and it's always sad seeing a couple like that split up, but you know, as everyone sees they're being very amicable about it and I don't really think there's any bad blood. It's tough. It makes me sad, to be honest."
Jenner previously commented on one of Carter's pics of herself and Cyrus from Friday, writing, "Hot girl summer," with a fire emoji.