Ugh, I'm so sorry. Siblings (and family in general) can be the worst. The most hurtful things said to me about my stutter have been from family members. Not just because the content of what they said was rude, but also because they've known me my whole life and should know better than to make such comments. Being that she cut herself off, she knows she was being mean, and didn't intend to be. In this case, it could be worth talking to her about it, and how shitty it made you feel. In other cases, the sibling is deliberately trying to be rude and hurt you, and talking to them about it likely will not do anything.