Don't worry, I don't even have a sister (or a gluten intolerance):
My sister poisoned me! That bitch! She swore those cupcakes were gluten-free, but I wouldn’t be sick like this if they were. She must have snuck some wheat in while baking them.
Why would she do this to me? Especially on my birthday! Is she still mad that I was a little late to her birthday dinner last year? She’d only been waiting 15 minutes, even if I was supposed to be there an hour earlier. It’s not my fault my seminar ran late. And why should I have left early if I knew she wouldn’t be on time either? Really, she got mad at me for not waiting for her for 45 minutes, and I apologized for months.
And then she poisoned me! I can feel the nastiness moving through my veins despite my body’s fierce attempts to expel it. I knew she wasn’t the nicest person in the world, but I thought she was better than this. A sister should be safe from the evils she unleashes on the world.
How could I confront her about this? I can’t just accuse my sister of poisoning me, but I can’t let her treat me like this. I won't stoop to her level with revenge. I’ll just have to tell her that I ate gluten somehow and that she’ll need to be more careful in the future because I know she’d never feed it to me on purpose. She’s too good a sister to ever hurt me like that and she must feel so bad about causing me all this pain through no fault of her own. So from now on she’ll just have to bring over only foods that are labelled gluten-free. It’s not my fault they cost two to three times as much as regular food. That’s just the price she’ll have to pay for having a sister who’s stronger than she thinks.