Why does Caffeine have a reverse effect on me?

There is a small % of the population that have paradoxical effects to a drug. This happens to me with succimone, the drug they give before surgery. The fact it happens to your sister as well suggests a genetic component. Of course, some people drink so much caffeine it has no affect - tolerance will negate any dopamine agonist. Actually, you and your sister should find some clinical trial to take part in, assuming neither of you drink caffeine at all. The pharmacological and genetic study findings would be interesting.