Oblena for giving your insights into this issue. I think you are very right, that this needs to be taken more seriously and action steps put in place to deal with this young girl, with the appropriate support and help.
I had a very troubled teenage life, my Mum left when I was fifteen and my Dad has what I now know is NPD and was incredibly emotionally manipulative. The thing was though, that he gave me no guidance whatsoever on what acceptable social behaviour was, so I have many regrets about my sexually precocious behaviour that was purely attention seeking. Looking back, I wish he had have laid down very strict boundaries for me - I'm sure at the time I would have thought it was incredibly unfair, but I would have been much safer. All I can say is thank God that was the era before social media.
I am a step-mum and my DH and I talked about discipline with regards to his son when we made the decision three years ago to move in together - he is younger than your girls, but I wanted to make sure we were on the same page. He has said that when DSS is in our house I am to parent / discipline him as I see fit and to pull him up on things if I feel he is out of line. Now having said that, DH and I have very similar views on parenting, so there haven't been issues so far, and DSS is a very good kid, so we haven't really had any challenges at this stage. But even though DH has stated that he is fine with me 'parenting' I am still very aware of the fact that I am DSS's step-mother and I do defer to DH if I believe there is an issue, and we discuss it, and then DH will deal with it, unless it is something quite minor that I feel comfortable addressing on the spot.
I think the important thing here is that you and your wife are united in the way you deal with DSD and that your wife takes the leading role as she should as her mother, with you being visibly supportive and showing that you back her all the way. Maybe it is something that you discuss with a counselor yourselves, so that dealing with DSD doesn't create a rift in your marriage, when you both want the best for this young girl. I think you have recognised some important issues - but may need some professional guidance so that you don't alienate your DSS and are able to give her the support she needs in a way that she is able to accept and relate to. Girls do need strong male role models that hold them in respect and if it is done well, you will have an adoring DSD for life.
I know when I watch my niece with my DH, it is the most beautiful thing in the world knowing that she has an uncle in her life who is a responsible, loving, firm, kind, respectful, open and communicative adult male that demonstrates with me, how a healthy relationship can be.
Good luck with your DSD.