13 year old step daughter in love with my 18 year old son?

I just found out that my 13 year old step daughter is madly in love with my 18 year old son about 6 months ago. We had a talk to both of them asking my son not to encourage the infatuation however my 13 year old step daughter didnt back away. My son found a girlfriend and it seemed to have stopped however just before Christmas my son broke up with his then girlfriend. Now I have found out that its started again, just from watching how she blushes when she sees him. We only have them 2 nights a fortnight but Im stressing as she is only 13. I have told my son that if she is telling him she is in love with him then he needs to put up boundaries as its not a good situation. I have explained the legalities as its in line with and she is a minor. I dont know what else to do. I find it wrong, They have been part of eachothers lives for 7 years and her dad and I have been married for almost 2 years. Help as I dont know what to do or say.