15 year old nephew has icky girlfriend - help!

What can an adult do to help a teenage boy understand that having a serious (and seriously inappropriate) girlfriend at his age is not a good idea? My 15 year old nephew “Joe” lives alone with my brother. Joe’s parents never married. Joe’s mom met a man online and moved across the country when Joe was 6, leaving Joe with my brother (who then lived with my parents.) Mom, who is now back in town and in another relationship is 44, dad is single and 54. I suspect all the adults involved could have alcohol problems. (I KNOW my brother drinks much too much. Mom “borrowed” and never paid back bail money from Joe a couple of years ago after being arrested in an alcohol induced domestic violence incident – an incident which Joe witnessed.) All Joe’s grandparents are gone now, and there are no real good role models in Joe’s nuclear family. Joe is a gregarious, smart kid, a good student who has a lot of potential. He has a college fund awaiting him when he graduates.

Joe met “Mary” in the 8th grade and now they are “serious”. On Mary’s facebook page, she has listed Joe’s mom as her mother-in-law. Mary posts inappropriate sexual comments, dropping the f-bomb on her page with regularity. Joe’s mom is facebook “friends” with Mary and seems to think nothing of this. I have tried to arrange family gatherings to which Mary is invited, but Joe has made many excuses for her not attending.

Now, to the question: First, is it any of my business who my nephew dates? Second, if it IS my business, is there anything I can DO, or SAY to Joe to help him understand that he doesn’t have to be so serious at such a young age? I try to be a good role model for him, planning activities and outings with my husband and me. I wouldn’t want Joe to know I don’t feel a warm fuzzy about his girlfriend. He clearly is smitten with her. I just want to point out his many options for the future. My husband and I have no children and I dearly love my nephew. Please advise. Thank you so much.