Married to Good Charlotte singer Joel Madden since 2010, Nicole confirmed that she would take all the credit for Cameron and Benji's new relationship.
"I'm going to take responsibility for everything" Nicole said to Watch What Happens Live on Tuesday.
Benji Madden with Cameron Diaz and a friend
When asked if she approved of their budding romance, Nicole said: "I approve of anything that's going to make Benji happy."
She continued: "I am a devoted sister-in-law. I'm happy for anyone who is happy, and I want everyone to be surrounded with love." She also revealed that the two couples had already been double dating.
Earlier in 2014 Cameron, 41, sparked rumours that she had found herself a new man when she was pictured out and about with Benji, 35.
The pair were seen hitting the gym in Los Angeles, sporting casual wear and looking completely at ease in each other's company. Cameron was later photographed dropping Benji off at the Montage Hotel in Beverly Hills where he was staying.
Benji later backtracked about their relationship after admitting in an interview with his brother that they were both "lucky guys" in response to the question of being in love.
He seemed to rewind when he realised his comments had caused a fuss and sent out a message suggesting his words were not to be taken in that context.
"When media take interviews & twist them into stories to get cheap web hits-don't let it get to U-just remember the music and stay positive," he posted on Twitter.
The couple then stepped out hand in hand in New York City in early June to confirm the rumours, and looked happy and relaxed and not at all shy about showing their affection for each other.
Benji is the first high-profile beau The Other Woman actress has had since splitting from New York Yankees player Alex Rodriguez back in September 2011. Her other exes include Justin Timberlake and Jared Leto.