Vev, I think you need to read more than the OP then, as that's not the case at all if you read through properly.
BrownSugarBabe - the step mother's dilemma! You've to be a parental figure to a child who isn't related to you (so you can't look at a specific attribute or form of behaviour and say that's just like so and so's, thus intsantly giving you an insight into why they're behaviing that way), and who has already spent some considerable time in another family unit where they do things quite differently to what you do.
But you know, if you belonged to some of the step parents groups that I do, you're hugely lucky to have such a wonderful bio mother. Build on that - more than once you've said how horrified she would be at SD's behaviour. So talk to her, say that SD seems to be having this sort of problem at your house, and what should you do? How would she like you to handle this? At the end of the day, she is the primary care giver and the person most in charge of this little girl's life. How this is handled will have a huge impact on this little girl, so do talk it over with the mum. She knows the little girl best. Just do it in a sort of 'I'm worried about this and wonder how you would like me to handle it' kind of way, so she knows you're not being confrontational.
I agree with what a lot of people have said. Much of this sounds like the normal sort of way a 7 year old might behave, so don't take it personally. Your son is obviously happy with his sister so don't go looking for problems that don't exist. He isn't for a second going to feel bad about not getting affection from his sister - he is surrounded by unconditional love so will just assume that she loves him. He is only going to start thinking otherwise if he picks that up from you. And I am sure that SD loves her brother - this will be great for him to learn that he can be loved in lots of different ways.
Also the advice on her having her own room - yes that will make her feel so much more secure. And it is OK for you to use the room for guests when she's not there. We use SS's room for guests and for drying clothes, and heck for general storage when he's not there. It is always put back correctly when he comes over. And when he was older we alwasy made a point of asking if guests could stay in his room beforehand.
I think that stopping her having visits at your house isn't the right thing to do. She is only going to get used to her role in your family by being a part of your family. And you know, it is OK not to like a step child now and again. Bio parents don't like their own child now and again. The key is to accept that this is quite normal and make sure that it never influences the way you treat the child. You're the adult and your only resposibility as a parent and that includes step parents is to make sure that the child feels loved. Even if you don't feel you can love that child, you have to make sure that you can do what you can for that child to be surrounded by the trappings of unconditional love no matter how hard that might make your life at times.
Finally, this might help. It is American, but these essays do give a good insight into some of the problems step parents face.
www.steptogether.org/essays.html