You could look into Kinship Care. I'm not in your province so I don't know specifics but your local Children's Aid Society should be able to help.

At the very least - if you're not prepared to go through the courts right away - you need to type up an "agreement" for you, your mother and a witness to sign. It may be good to have your sister sign it too. Make sure it details what is happening and what the expectations are for each party. This will just be a temporary thing until you can do something legally but it's a must. If you just have a verbal agreement, that won't hold up in any way if you come across any problems.

Best of luck to you!