Don’t do it! I will just end the same as last time. You broke up for a reason and the reason is still there and the issues are too. 

I think you do know all of this.

If it’s real, it should be a ‘no’ to going back to this. It’s hard for me to understand how their dynamic wouldn’t turn you off to the point of no return. 

They have a very unhealthy relationship. I say go to the counselling and tell the  counsellor everything, the dad needs help, he needs to realise it’s far from normal. If he can’t see it and change after a couple of sessions then run run run! 

You said at one point in your post "Dad created a monster" - would you really want a relationship with the creator of such a monster?   If you do get back with him, for heaven sake keep your own place, don't move in with him.  

Walking around in a g-string in front of her dad? And he thinks this is normal? What is wrong with these men???

It’s a big NOPE from me, you’ll just end up with more of the same, where you’re constantly in ‘competition’ with the daughter & daddeeeee thinks it’s totally ok. Not worth it, you deserve better & you can’t fix them.

This is really really extra-sickening. If you won't trust what your own guts are telling you, at least respect your son enough to never go bsck. 

Dude, he's already had at least two women leave him because of the twisted relationship, lack of healthy boundaries, and refusal to parent his daughter.... what makes doing this again appeal to you? 

The very definition of insanity is this: doing the same thing over & over but expecting different results. 

If you're honest with yourself, you'll walk away from this unchanging scene. Peace to you.

Bluntly, I have no idea why you would even consider going back. 

May you air your grievances and get enough feedback on here about this incestually-driven, sexually-charged daddy-daughter duo to LEAVE THIS IDIOT ASAP.  You've already experienced the emotional see saw and cut it off because you KNOW it's not going to last. 

MIss teen thong and daddy dipshit are not going to change their game because of you. If change was possible/probable then there wouldn't already be a couple of ex GFs who underwent the same dysfunctional game-playing. Don't give them the satisfaction.

The issue will continue to escalate as the teen gets older. The incestual energy is brazen. Something about his sexual energy is distorted and don't be surprised to find out that you (and all his other ex GFs) are merely an outlet that is socially acceptable. (Ask your psychologist daughter about this.) A grown woman in a relationship should NEVER have to hide from a jealous teen. Do you SEE this behavior for what it is? It's as if though he is cheating on HER with YOU. YOU are the "other" woman.  Some part of him enjoys feeding this warped behavior. 

Normal fathers see their teen in a thong and say things like..."put some damn clothes on OR you better not be wearing that in public."  (And most teens, although they may dress risky will not strut it in front of their fathers...they'll wait to change in the car).  Decent, concerned fathers are protective and respectful of their daughter's sexually maturing bodies and minds. This guy is only playing the "good dad" role when you are around to appease your discomfort. He is silently advocating her sexually provocative behavior.  

Aside from what everyone else has already posted....what part of you wants to stay remotely invested in such a relationship?  Be honest with yourself.

Leave & call your version of social services/child protection, it’s the only way they’d change. Just report them for possible incest explaining their relationship and your concerns. Let the government workers explain this isn’t normal. Save the next woman for this crap! 

The answer to your question is in your own post. "He says she's over it now." But later you write that SD threatened not to see him if he sees you again.

Right here is your problem: he's telling you something that clearly is not the truth. Look at the evidence, not what he's telling you. He either doesn't see her as she is or he's hoping you won't see her as she is. Either way, what she is is what you're in for.

As they say when you're lost at sea on a tiny raft...pray to god, but row for the shore. Have your fun with him, but keep looking for a better man because this relationship is open water and your rickety raft has already made this trip once.

I've been down the road of weirdo erotica between dad and daughter. (I remember SD's red thongs left on daddy's bathroom floor after her showers.) It may lessen, but the SD hostility and drama won't. That is who SD is. If you don't like her that way, there's only one way to avoid her.

Oh, Jesus, really?

At this extreme I'd say it won't change.  That said, some DO come around with proper boundaries placed by dad or when they start having their own lives.  SD was ultra creepy/enmeshed also and ages 10-14 were the toughest.  She did outgrow it and has her ow boyfriend/life and is hell of a lot more proper around DH and I.  But BOY did we have some struggles.  Difference is when I pointed stuff out he'd recognize it as wrong and try to cut the behavior out and created boundaries.  Wasn't an easy road.  And your daughter is right..it's easy for single dads to get into this role w/their daughters-they need a clear example of couple/united front.  

In an intact home MOM teaches proper behavior to her daughter around her father, brothers, uncles- about being modest, covering up, etc....but if a stepmom dares try she is crucified for meddling, being jealous of a 'child' and a bunch of other nonsense.  Intact homes mom corrects this all the time.  I think your case is way too extreme though I'd run for the hills and never look back.