What should I expect when being visited by Children's Aid Society of Toronto?

Tell them everything you have said here. They have to investigate every report they receive so it's nothing personal to you if they ask a lot of questions. Tell them your sister is doing this because she is unstable and malicious. Also you should ask them if there is any recourse you can take for your sister falsely charging your father and putting your child through this. Your sister has no right wasting the time of people who are trying to help abused children. I suggest you get a restraining order (actually i think your whole family should) and seek some justice. Just be open and completely honest and tell your kids to do the same and it will be fine they will see that your sister is nuts.