I would really appreciate some advice/ input into a bit of an issue I have.

17 year iold stepson has been living with us for about a year...has had his issues but is overall a nice kid. He has a girlfriend who he has been seeing for about 8 months now.
She is a nice person, very very shy, but is always nice to my younger kids and doesnt cause any trouble when she comes to visit...but my main issue is her complete lack of common courtesy.
Now as I said she is very shy, and will generally not speak to hubby or I unless spoken too. But the issue I have is that she has NOT ONCE thanked me for feeding her, hosting her in my house (ie: if shes stayed the night, no "Thank you for having me"....just heads out the door without so much as a goodbye even!)...and another thing that irks me is that when she comes over, she doesnt acknowledge anybody in the house with a hello, how are you...just heads straight up to DSS's room.
As far as Im concerned, if you are entering someones house, you at least offer a greeting to the host and acknowledge that you are in their house, instead of walking past them as if they are the hired help. And if you are getting food and drink and use of amenities/ shower, as she is, you at least say thank you at the end of the visit.....but instead, she leaves it up to DSS to say it for her. I even saw them play wrestling when DSS was trying to get her to say thank you for serving her dinner, but she was laughing and saying "NO, I dont want to". So in the end he said it for her. If its late at night or bad weather, either me or hubby will drive her to the station to catch her train home (stations about a 2 minute drive from our house and she she lives about 50 km away in a semi rural area, so limited public transport here)...nope, not even a thank you or goodnight there either!
Like I said, dont mind having her over to visit DSS, but this lack of manners is really getting on my goat. Shyness is no excuse for not offering some sort of acknowledgement of what you have recieved, IMO....and its getting to the point where I am about to tell DSS that unless she starts acknowledging the trouble we go to for her and starts displaying some common courtesy, instead of bypassing me like Im the servant around here, then she will not be welcome in this house. I have taught my younger ones, from day dot, that good manners cost nothing and it is the right thing to do, and as a result, people comment on how well behaved and polite my kids are. But I refuse to be treated like the hired help in my own house by DSS g/f......shyness is no excuse in my opinion. Ive never had this problem with his other mates..they have always said hello, goodbye and thank you...very polite young people.
I would like to know if maybe I am being a bit precious about this? I am extremely hormonal ATM due to being 25 weeks p/g, so maybe I am taking this a little personally, but in society today when people seem to have forgotten common courtesy, and I am trying to teach my children not to go in that direction, I really dont think that a please and thank you in my own house is too much to ask for.
Does anyone else have issues with teens and manners, or lack thereof?