Why does my kitchen sink fill with clean water from the drain when the toilet upstairs is flushed? It also kicks the sump pump on in the basement.

Is your sump pump tied into the house sewer? Sounds like it is. My best guess would be that your house drain is partially plugged, your sump ties into the house sewer, and the sump pump either does not have a check valve, or the check valve is faulty. If this is the case, when you flush the toilet, the water is backing up into the sump pit(do to lack of check valve) causing the pump to kick on which is sending the water in the sump pit up to the next highest fixture (presumeably the kitchen sink) since it can't go down the house drain. Any of this making sense, or am I way off the mark? More details please.