I'm guessing that her sister had some psychological problem, and also engaged in destructive behavior, including bad eating habits. She she sees the comment about getting your head read, and connects it to her mentally ill sister, and then on top of that, you also have these self-destructive habits like eating ice cream for lunch
Throw in the fact that you're joking
about it, you're clearly very very sick, and not taking it seriously, and therefor, you're going to die like her poor sister.
I know she was way out of line, and way off the mark, too, but I do feel sorry for her. Imagine how the loss of her sister must have impacted her, if she's reacting this way.