Sally, sometimes people think others are doing things to them when they aren't. Water that isn't boiled isn't poisonous --most people drink water from their taps and don't boil it and are very healthy. Sometimes things do get into the water supply system that make the water unsafe, but the officials keep a check on that and will notify the community. If you drink water from a well it's probably good advice to boil it. People also get sick for many reasons including usual flus and colds, so you may have just gotten an ordinary sickness and thought it was from something your sister did. You should never try to poison your sister or try to hurt her or someone else. When people are over anxious about an issue like this it's best to find a good therapist to talk about it with. If you aren't seeing a therapist now and don't know of one, ask your regular doctor to recommend one to you.
