Oh, girl, what an awful situation! I do understand that you do not want to cause hard feelings in the family, but only to a point. Respect is not a one way street and it looks like she has some lessons to be learned in that area.
Look at it like you are doing her a favor by helping her to learn what she needs to know. It will not only benefit you, but her as well. She is young. Tell her you do not want to hurt her feelings, but....and then enumerate the problems that happen because she needs to learn these things. All the people in your house have a right to be free of the distress she causes by her ignorant (as in unlearned) actions. Geesh, they are feeling like running and hiding is their only option.
If you do it gently, instead of waiting until you have reached your wits end (and maybe blow up), then there is a possibility of growth for her. Growing is stretching and stretching always brings some discomfort. Why should all the discomfort be yours? Which it currently is.
I hope you are able to get some relief from all of this, but the only way that is going to happen is if you meet it and deal with it directly. How awful it must be for you to just keep this stuffed inside yourself.
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