Questions- 3/6/15

Hello everyone. So today was the first day that our oldest Tessa asked me why her sister had seizures and asked what they are. This stunned Eric and I. Not is the aspect that she asked just in a way on how to explain it to her. Tessa is 4. She has seen and heard so much the past year on all the different things that her sister has to deal with. So my question is for all you who maybe reading this... How would you explain this to your 4 year old. We tried to the best way possible but was unsure of how to explain it the best way for her to understand. Any suggestions would help. Tessa does understand that her sister is different and has to have special attention because of these needs. But we do our best to keep her involed and try to explain things the best we can to her. 

Thank you for the help! :)