Did your sister actually hit you with a closed hand(I think an open hand counts in Canada aswell)? If so, thats assault, regardless of how strong she is or not, she is grown enough to know not to lay her hands on another person. There is also no excuse whatsoever for your mother to begin yelling and shutting down the conversation when you bring this up. You have a rational complaint about a responsibility that isn't justly yours. How do you bring up this issue to them though, are you calm and collected or emotional when you bring it up? They have no excuse either way but I'm just trying to help you avoid conflict with them. Explaining to them more why it isn't your responsibility also will not work as they have both already showed that they have no respect for rationality.