Dear Not Really Stepdad,
You are asking to do what is promised by half of pornography, and is not-so-secretly acceptable in this society.  The fantasy of an older man with an 18 year old girl.  Unfortunately, this would probably cause irreparable damage to both Ariel and Wendy.  Whether explicitly stated or not, you are a father figure in Ariel's life.  She probably has a bit of Electra Complex going on.  If you pursued any kind of relationship with her it would lead to an inability to form healthy relationships, as well as depression, drug abuse, prostitution... well, maybe not that far.  It would definitely ruin any chance her and her mother have of reconciling.  Basically, you get to walk away with the fading memory of one (or maybe more) night with an 18 year old girl, and that family is destroyed.  If you lover her, set her free, if it's just physical, find a dating or porn site and move on.  Also, break up with Wendy, there's clearly no future there for either of you.