His FORMER stepdaughter comes to your house? How long WAS he her stepfather? If she has an involved father, why is he continuing a relationship? Was he in her life for a long time? 

This behavior with a non related person would creep me out too. Why is he driving her to school still? She has two parents, he is not one of them.

I think you have to listen to your gut. How old is this girl?

That is odd. He could catch lunch with her or go to a movie with her with you and the rest of your family but why is she coming to your house? She has "mini wife syndrome". Please read about it and see if that's what you think it is. Its so weird and sickening. I know from personal experience!! 

His biological children you are stuck with and have to find a way to deal with if you truly want to be together.  Even they can be dealbreakers for a marriage.  But some random girl that he is not related to and broke up with her mother 8 years ago?  Who is now 16 and is trying out her budding womanly skills on?  Hello Lolita!  If there really was nothing to the issue then he would not be so defensive and go immediately to the "sick - you need help."  He would look objectively at the situation and address your concerns calmly.  Jumping to the jealously is a major "tell."  Not that there is necessarily something physical going on but that there is an element of attraction and the forbidden blooming.  It is basic biology and the additional element of 2 women "fighting over him" is only going to add to it for him.  Why would he cut off that ego booster unless he absolutely had to???

This would be too much for me.  You are never going to be comfortable with her hanging around.  Even after she moves on to having a normal relationship with a boyfriend, you are always going to be on watch for flirtation between them.  And if you manage to ban her from his life, you are going to deal with his resentment or perceived resentmentm especially as you get older and wonder if he is looking for someone younger to have a fling with.

I would have one more attempt to have a real heart to heart with him on the subject.  But if he can't understand what is going on and how this is affecting your relationship, I would have to come to the conclusion that he is not in a marriage space and maybe never will be.

So many red flags and lines crossed here. I find it odd that your fiancee maintains such a "close" relationship with his step daughter who has her father actively in her life. He's been allowing this for so long that he sees nothing wrong with it, she doesn't either and he's doing nothing to correct this. She's jealous because you are occupying his time and instead of him addressing it, he just lets it fly by.

I would reconsider marrying this man. He allows this child to disrespect you and expects you to just suck it up. He doesn't take your feelings, wants and needs into consideration and that says a lot. If your fiancee doesn't change please know this will be your life, competing with a teenage girl and with a future H who has no boundaries or respect. 

Do not marry him. 

You should be able to bring up your concerns without being gaslighted and told you are sick and need help. Honestly, that is emotionally abusive for him to say to you in ANY situation (unless it's true and he's saying it in a loving way).

Please don't marry him. I promise you will be miserable. You say he's "good to you", but this is abusive and NO abuse should be tolerated in a romantic relationship. 

Gross. I’m sorry, but I don’t see how you can be attracted to him with this dynamic at play. It’s inappropriate behaviour on both their parts, but HE is the adult. HE is allowing this. HE is continuing to have a relationship with an ex-stepdaughter, gaslight you, tell you that you’re sick because you’re taking issue with HIS behaviour. 

Please do not marry this guy. And for heavens sake stop allowing this disrespectful brat into your house. If your FH gets angry that’s all the more reason you shouldn’t marry him. He shouldn’t allow any child in his care to be disrespectful to you, especially in your own home. SMDH.

Listen to your gut feeling on this. If she is 16, unrelated to him, and acting this way, there's a chance he may be actually molesting her. This behavior coupled with the ages he has been with her make me wonder if her behavior is the result of her being the victim of a crime. That's the worst case. Best case, he is an icky douchebag who isn't setting limits because it strokes his ego. Could you be attracted to that long-term?  

I noticed at the end of your post you said your son likes your FH but your daughter did not.

That statement coupled with your gut telling you there is something wrong with your FHs relationship with his SD should cause you serious concern. 

Maybe your DD sees this behavior as creepy as well.  Has he been too flirty or touchy with your DD when you weren’t around?  

Im just saying if your own DD can see he isn’t ok and your gut says there is a problem....there is a problem.   

Absolutely everything SM12 has said is what i was going to.

Alarm bells would be blaring esp with my own dd in the house. I would be checking at a much deeper level *why* my dd didnt like my partner. Lets hope his sick behaviour hasnt oozed into your dd's direction*bad*

Your fdh is treating his interactions with sd16 as if shes the OW. Deleting messages -an action when there IS something to hide. Furtive, lingering glances between fdh&sd -bleurg.? D/O with sd last-why? Competitive, jealous, rude behaviour from sd towards you. Acknowledgment by fdh that sd resembles her mother who he used to bone, an ex who spurned HIM. 

I wouldnt allow this probable predator to lie and manipulate you too, get rid and fast.

Everyone here has already said everything I was thinking. Please update us with your decision on what to do. I really do worry about your own daughter in this situation as well.