Sorry. I can't relate with this one very well because I have not experienced it. BUT, there are a couple of things you said that I would like to point out to you:
"We are practically like sisters" but you are not sisters. And even sisters fight or tell the other one off when it is appropriate.
"I'm just standing there looking at them" because you cannot tell them how you really feel because... ???
"because I don't know what to do" Yes. You do know what to do. You are afraid to do it. You want to tell her off and you want him to tell her to knock it off.
"I can't stand it." Yes. Eventually you are going to blow your top. Someone is going to pay. Either you, her or him. You are already paying. So, that leads to #5:
"I am really hurt." Because you are paying a price. Because you are afraid. Because instead of "hurt" you are ANGRY. Anger is healthy. There is a reason for anger. It tells us, "What is it that is unfair?"
Whenever you feel anger, ask yourself: What is it that is so unfair? Then ask, Am I being realistic?
In your case, yes. You are being realistic. But if you wait until you blow your top, then you will not be realistic in you approach.
You do not need anger to justify standing up for something you believe in.
Just tell them that she is a big flirt, he is your boyfriend, it makes you feel sad and angry and to please stop doing that anymore. Get them to promise you that they won't.
You could say: Ummmmm.... I really need to tell both of you something right now... together... and it isn't easy for me because I love both of you... but, ummmmmm
"girlfriend" ...I need you to stop flirting with my BF and ...BF, I need you to know that it makes me uncomfortable, sad and hurt when both of you are together like that. I feel disrespected and humiliated by both of you... so, it would be great if you two will respect my feelings and stop flirting. I want to trust you, but your behavior is not trustworthy. No one in a solid relationship flirts with someones boyfriend. I am being realistic. It just isn't something I should be expected to trust.
You can use your own words. You get what I mean, though.
If you don't say something... and then you find out later that they had an affair... you will regret not saying anything. Trust... but verify.
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I'd kick my sister out of the house if she flirted with my man. My best friend? I'd kick her to the curb. There's nothing wrong with her liking him, but the fact that she'll disrespect you in your face by flirting should send off some sort of bell or whistle. I'm assuming you're over 18 and in control of your own life. So, stand up and take control of the situation. Invite your friends to lunch, tell her what you've seen, how it makes you feel, why you think it's wrong, and that you aren't going to put up with it, because fiends don't flirt with friends' men! Then I'd let my man know what transpired and tell him that if he reciprocates AT ALL, you'll be free of two people who obviously don't care about your feelings.
Most people can't actually swallow a pill this big, but the reason we are treated poorly by those around us is because we allow them to do it. You'll find that when you rid yourself of the people who put their wants above the friendship/relationship with you, you end up happy and with people who aren't out for self.
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I find it interesting that you still consider her your best friend when she's openly flirting with your boyfriend. That says a lot. It is also evident because you have yet to say anything to her. I don't know if it's fear, or some deep seated loyalty that you have to her. A true friend wouldn't do what she's doing. You need to bring that to her attention. There's no way that your best friend should be flirting with your boyfriend. Your boyfriend is probably waiting for you to say something because that's your job...she's your friend! He's expecting you to say something about it to her.
Stop being so weak and passive! If you really care about your boyfriend, then you need to show him and you need to show her. Speak up and be heard. Let them know that it will NOT be tolerated.
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