Simpson and Mayer have reportedly been an item for the past eight months, despite initial denials they were dating.
The couple recently enjoyed a romantic vacation in Rome together and Ashlee insists her big sister is very happy with her new man.
Meanwhile, celebrity pal Christina Milian is thrilled for the couple: "This is a relationship that's not for the cameras. It's not about celebrity. It seems like they truly have an attraction to each other."
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