The 29-year-old Australian appropriately captioning a third post, "#twinning" on Instagram. She also shared it on Twitter, writing, "So this happened.. @justinbieber haha my brother."
The two warmed up their budding friendship on social media earlier in the week when Justin shared a photo of Ruby playing a piano covered in plates -- with his face on them! -- to which she replied, "You're welcome to use it anytime."
Justin was in Vegas for the Electric Daisy Carnival, a weekend-long EDM festival, and he made his presence very well known.