My dear friend,
I am writing this letter as a reply to u to handle the situation with your elder sister. You should say her how you feels when being teased, and ask her what would she do if this happen to her. Tell her how an elder sister should be and how well should she be. If anything happen to her, you'll be the only one worrying about her in the place of a sister. Forgive her this time, and if she repeats it, tell to me, I'll handle it.