I don't think you're being precious. I take it she's been coming over for about the length of their relationship? So she's been coming to your house for around 8 months and still hasn't gotten over her shyness enough to say thank you and acknowledge you?

That seems kind of strange to me. I don't care how shy someone is, she isn't so shy that she stays home all the time and she has been able to commence and maintain a r/s with your DSS so she must have some social skills.

I think you should sit her and DSS down and discuss it. Tell her the effect her behaviour has on you and the rest of the house (feeling like hired help etc) and talk about how she's going to have to deal with various people as she grows up/becomes an adult and being able to use common manners is just one of many life skills she needs. She should be practising with you and your family in a non-threatening environment if it's that much of an issue for her.

If talking to her doesn't change anything, I think you'd be within your rights to say she's not welcome in your home. That's a tough call because she does sound like she doesn't cause any other dramas but it's your home, guests should respect you and your rules. GL with it.