I don't want to sound like your parents, but I do have to SUBTLY agree with them on one thing. Almost all siblings fight. Siblings all say things that they don't mean, and typically they feel bad about them later. At least that is what I have learned from experience. Even though your brother is saying those horrible things to you, you know as well as I do that if you were to do something bad to yourself, he would feel nothing but sadness. You know that, that isn't something that he means, and he is just saying it so that he can attempt to say something that will actually get to you and potentially emotionally hurt you. He has no desire for you to actually physically hurt yourself, this is all an emotional thing with him.
I know it is really hard to look at it this way, but did you ever consider that your siblings are treating you this way because they care about you? I am in NO WAY saying you are a slut, and I am by far from saying what they said was right, but the truth is that no one wants to see their little sister dressing provocatively. Even though they are going about it in the complete wrong way, isn't it possible that they are just trying to deter you from the lifestyle that they don't agree with? That is a definite possibility.
Now, there isn't really all that much that you can do to get them to stop. As you said, your parents are of no help to you, and tbh I don't think it would do all that much good if you were to tell your siblings how you feel. I know it sounds so cliche, but keep your head up. Don't let what they say get you down. When they are saying mean things to you, try and keep in mind that it is most likely because they care about you. I know that is SO hard to always think about when you are being brought down, but it is stories like this where the term 'tough love' was coined.
I hope things get better for you, love.
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feel free to demote me, I'll be inactive.
lalalala
bai