Vanessa Marcil, who shares one son with Brian Austin Green, wrote "#ImWithYouSister" on Instagram after Megan Fox called out the actor — her ex — for painting her as an "absent mother"

Brian Austin Green's ex Vanessa Marcil appears to be siding with Megan Fox after the actress slammed him for portraying her as an "absent mother."

After Green, 47, posted a photo of himself and his and Fox's 4-year-old son Journey River on Instagram for Halloween, Fox, 34, wrote a lengthy comment questioning why Green publicly shared a photo of their child.

"Why does Journey have to be in this picture? It's not hard to crop them out. Or choose photos that they aren't in," Fox wrote. "I had a great halloween with them yesterday, and yet notice how absent they are from my social media. I know you love your kids. But I don't know why you can't stop using them to posture [sic] via Instagram."

Fox continued by adding that Green is "so intoxicated with feeding the pervasive narrative that I'm an absent mother, and you are the perennial, eternally dedicated dad of the year."

"You have them half of the time," she wrote. "Congratulations you truly are a remarkable human! Why do you need the internet to echo back to you what should be inexhaustibly evident in the way your children love you?"

Vanessa Marcil, Brian Austin Green and Megan Fox
| Credit: Paul Archuleta/FilmMagic; Jonathan Leibson/Getty; Steve Granitz/WireImage

Marcil, 52 — who has previously spoken out in support of Fox amid the star's split from Green — wrote on Instagram Sunday, "#ImWithYouSister," though she didn't mention Fox by name. Marcil, who shares 18-year-old son Kassius Lijah with Green, added: "Like I said... the truth always comes out in the end..."

Megan Fox, Brian Austin Green
Credit: Brian Austin Green/Instagram

Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer, from juicy celebrity news to compelling human interest stories.

While it’s unclear why they split, Green and Marcil called it quits shortly after Kassius' birth, in 2003. Green began dating Fox the next year. In 2010, Green married Fox, and the pair share three sons together: Journey, Bodhi Ransom, 6, and Noah Shannon, 8.

Green announced he and Fox separated late last year after she came back from filming a movie overseas and asked for space. She has since moved on with boyfriend Machine Gun Kelly. A source previously told PEOPLE that Green was "annoyed" that Fox moved on so quickly with Kelly, 30.

In September, Marcil wrote on her Instagram Story after a fan asked about her thoughts on Fox that she admires the actress. "I actually have respect for how @meganfox is finally living her life for herself & leaving her children out of her public life at this young age," she said at the time.

"I used to hide Kass from the public when he was young & never once spoke about our court case to kass or publicly until Kass was grown & insisted I stop lying to protect," added Marcil. "Megan finally living her life for herself is in itself a gift to her children."

RELATED VIDEO: Brian Austin Green 'Upset' Wife Megan Fox Has 'Moved on So Quickly' with Machine Gun Kelly: Source

In late August, Green said on Instagram Live that although he and Fox are on different paths at the moment, it's impossible to tell what the future holds. “I never say never,” he said when asked about the possibility of one day getting back together.

“I kinda feel like people are on paths in life and sometimes your paths are together and you travel that path together and you see eye to eye and then sometimes paths do different things,” he said.

“We had an amazing 15-year relationship. We have three beautiful kids. We shared a lot together and we really went through a lot together,” Green continued at the time. “Right now the paths are different and she is on her path doing what she feels she needs to be doing to be happy and I’m on my path doing what I feel I need to do to be happy and it’s not for a lack of love for the kids or lack of being responsible.”