Inappropriate Teen

[IrishDU]

I knew when I started dating my new gf that she had a 12yr old daughter, that for more than a decade they only had each other, and were basically joined at the hip.

My partner is a stunning beauty, several years younger than me, from a different culture, and I consider myself the luckiest man on earth.  Hee daughter is basically a little clone, and they share a lot.  Personally I think my GF treats her a little too much like a young adult, but as I said different cultures and besides they are her decisions as parent, not for me to criticise.

I learned pretty quickly that she is precocious, outgoing, and had no boundaries.  We'd barely began dating when her daughter started calling me "papa" and had gone through my facebook and introduced herself to my kids (who are all older and adults now.)
Now I love them both, but there are aspects that I find very confronting and quite frankly disturbing.  And at times, I worry that she has gone beyond simple "charming precociousness" and is deliberately pushing boundaries to get a reaction.
It would be easier to read is she wasn't just such a great kid and joy to have around.  Plus there is still a cultural barrier, and a slight language barrier that they both love to duck behind.

When I first visited my GF, I was very confused (but ultimately a little relived) that her willingness to let me see her undress, naked, and even bath with me, did not equate to a similarly casual attitude towards sex.  This attitude seems to be cultural, but I still find it very disturbing.
The first time we went to their beach, both she and her daughter, wore nothing but a "string" bikini bottom (and even the string was removed for tanning purposes.)  Needless to say that after that I decided their beaches were too "cold" for me.  And where we live now I've convinced them that the sun is too dangerous.

We've now been together for a while, and we've both come to refer to her as "our" daughter, but I still respect my partner's role as her primary parent.  
I just don't know how to deal with the more inappropriate aspects of her behaviour.  Unfortunately my partner just thinks I'm a puritan and makes a joke of it.

She is still not even 14 yet, and I'm worried that her attitude may get her into strife.  
My partner is an incredible woman, but yeah she got pregnant to a boyfriend who disappeared from their lives shortly thereafter.  And when she tells me that she was already having sex at her daughter's age, and that she will have sex when she feels ready for it, that really upsets me.