And that bond has certainly played a big part lately in helping Bruce Willis and Demi Moore's 23-year-old daughter deal with the troubles her mother has had recently, which culminated last month with Moore checking into a rehab facility.
So when E! News asked Zac how Rumer is holding up, here's what he had to say.
"Yeah, she's doing OK. She's doing fine," the actor revealed during a press junket for his upcoming animated flick, Dr. Seuss' The Lorax. "It's tough, but I think she's all right."
Here's wishing Rumer—as well as siblings Tallulah and Scout—all the best during this difficult time.