My boyfriend and I both are 17. I have recently found out that one of the girls my boyfriend introduced to me as his sister is not, in fact, his sister. She is just a really close family friend. She has no blood or legal ties with him what so ever! Yet she is so close to his family that she has her own room in their house with her clothes and stuff. Apparently, she stays there whenever she feels like it, which is 2-3 days a week.

She calls his mother and father mom and dad and behaves like she is a part of their family, except that she is not. She lives with her uncle and aunt who are old friends of his family and allow her to do whatever she wants. She calls my boyfriend whenever she wants to go somewhere for a lift and even tells him that she loves him. I find it so wrong on so many levels that I cannot even begin to explain it. His other two sisters are somewhat like her too but they are his real sisters not fake ones. I tolerated her shenanigans with him just because I thought she was his sister, but now I find her unbearable.

Last week I went to see him at his house and she opened the door wearing his shirt! And everyone was cool about it like nothing happened. I literally cried after I got back home. The thought of her staying in his house and going into his wardrobe trying his clothes on makes me want to cry with jealousy.

When I talked to him about it he said that he considers her his sister just like his other two blood sisters. That's why he introduced her to me as such. I have nothing to worry about. But he will not stop her from stealing his clothes and kissing him goodbyes. She even steals his dad’s clothes, no one can stop her. He says that she has been there since they all were in kindergarten so I shouldn't expect him to shun her away and that I am being irrational.

What do you think? Is their relationship not objectionable? Am I not justified in demanding that he distance himself from her? I may not be able to make her stay away from his home because she unfortunately has a relationship with the whole family but at least I should be given assurances that she will not enter his room or wear his shirts and hoodies? Or jump on him or call him like he's at her beck and call? Why can't he just rebuke her so she backs off? Is this really too much to ask for?

PS. She is 17 too.

First of all, anyone in a relationship has no rights to 'demand' anything. When you encounter something you don't like, you express your feelings, and based on the response, you accept it or LEAVE.
Second, it's very common for people growing up together to develop a total inability to see each other in a romantic or sexual way, such as all the kids who grew up in kibbutzes in Israel seeing each other only as siblings, even though they weren't.
You need to do some reading, and you need to understand more about where she is coming from in terms of perhaps a rough life without her biological parents.

Agree, you can and should not demand anything, you either accept it, or break up and find someone new.

And this is more common, esp in the south, where a non family member lives in a home and is raised by a family. And to them, she is a sister, I am sorry if you can not see it, but everyone else does, the only problem here is your conception and perhaps narrow minded ideas that sex has to happen. No difference than a step daughter, that would be no real blood relation living there.

I find your reaction very selfish, and his and his family loving and accepting. You need to lgrown up and learn something from them.

The only problem I see here is you. It was probably easier for him to introduce her as his sister because that is how he views her. Siblings often share clothes, what is the big deal? Siblings love each other. Siblings give eah other lifts and help where needed.

If she was his bio sister would you still object? Because for all intents and purposes they consider themselves siblings. So you need to back off or end the relationship.

He should not shun her because you have insecurities. How long has he been your boyfriend? Not as long as she has been his sister I bet.
If you don't like the relationship, you should leave.