My girlfriend wants to let her brother live with us. I don't feel comfortable with that.

My girlfriend and I moved in together two years ago, and it has been great and all that, though I am having some trouble with her family lately. My girlfriend is 25F, and her 23M brother. We all get along well with him, and he is a good guy, though my feeling of discomfort stems from their weird relationship. As I am not sure how else to put it, I think it's somewhat inappropriate to say the least. They have a very close relationship, that borders on the line of inappropriate. They have told me that they consider each other their best friend, and they hang out together daily. I can be understanding of that, though I feel uncomfortable with this level of closeness. I have grown up in a big family, so I have many siblings, and none of us are this close. Whenever they hang out together, they are always physically touching each other. Either by hugging, kissing, and they usually sit very close to each other. They have told me they grew up in a very loving environment, and they have been encouraged to hug and kiss each other, and they take after that.<br><br>I personally can't stand physically close people in my personal space. As I am not sure why, I feel extremely uneasy even with the thought of being hugged, and I can't stand anyone meeting my personal bubble. So to see them always being so close to each other, makes me feel very uncomfortable. I have spoke to my girlfriend about this, and she has told me that she can't help it that they are very close. So I have had to somewhat deal with it.<br><br>Now their mom is going on vacation for three months, so she asked if her son can live with us. Now I am not against the idea of him living with us, I can actually tolerate him as a nice guy. Though I feel uncomfortable with the idea of him and my girlfriend living together because of their closeness. We have a big house, so they'll have their own bedrooms, though how will we keep him from visiting our bedroom and how will we have alone time together. I can't tell them not to because that would be unfair to them right?. So I told my girlfriend I wasn't comfortable with the idea of him living with us. She got upset with me and told me she felt I was being unfair to her brother and asked me why I was being this uncomfortable. So I told her I wasn't comfortable with the level of their closeness and that I don't want to have to constantly see them touching each other. She told me that's unfair of me to ask them to stop showing their love for each other, and refused to talk any further with me. I feel like I am being unfair to them but at the same time I feel uncomfortable with them in the house.<br><br>&#x200B;