The 21-year-old singer's grandfather Ricardo claims she is trusted to be a "good influence" on the troubled pop star - who was arrested for driving under the influence last year - and while they are no longer dating, they still hang out.
Ricardo said: "She was telling us that she didn't want to be with him because of what he was doing, that he was getting into drugs and it was something she doesn't like.
"They still go out. We worry, not just because of their relationship - we hate to see him in trouble.
"The agents say it is good for her to be next to him, keep him out of the bad habits. They say she has been a positive influence for him. She's doing all she can to get him on track."
Ricardo also opened up about his granddaughter's two-week stint in rehab in January, which was thought to be for exhaustion, but he feels Selena's issues with Justin contributed to her seeking help.
He told Heat magazine: "There was some pressure because [her career] is a lot of work. Plus, she was worried about Justin.
"We saw in a newspaper that she was doing drugs, so I called her. She said, 'No, you know better than that, Pap.' "