His sister Alli Simpson said: "We love her. Cody is very happy and it seems that Miley is, too. We're always jumping on FaceTime with them. They always just seem so happy and I always say, if Cody is happy, we're happy. That's how we've always felt about our partners in life. If they're treating us well and we're treating them well, the family love it."
And the Pretty Brown Eyes hitmaker recently gushed about how "happy" he and Miley - who split from husband Liam Hemsworth earlier this year - are together.
One of Miley's posts from her Instagram story. Photo / Instagram.
He said: "I'm very happy. We are very, very happy. She's creative. She's very passionate about what she does, and I'm very similar in that sense. That's why we get along so well. The reason why it hasn't been a really crazy sudden thing is just because we've been friends for so long."
He added that they "found each other again in a space where we're both not partying, working real hard, and it just keeps things healthy and it's good."
Miley Cyrus has sent Cody Simpson a birthday message on Instagram. Photo / Instagram.
And admitted their friendship has been "developing naturally into something more", he continued: "Any girlfriends I've had in the past has always been immediate romance and so it's a different experience having been friends with somebody before.
"It developed naturally into something more. We just have a ball, yeah, and that's the most important part of a relationship, you know?"
