You could not have applied for your citizenship at age 11 nor your sister at age 14 as you were both minors. A minor can not apply for citizenship for themselves. Their parent must apply for them and their parent must either be a citizen or be applying for citizenship at the same time.
Now that your sister is 18, your mother can not include her on her application if she applies again. She can include you though but your sister will have to apply for herself. They still have to pass the test though. Tell them to go to
http://www.yourlibrary.ca/citizenship/ and pick their province, pick "all" questions and practice until they get them all right.
If your mother doesn't succeed, you can apply for yourself when you turn 18. Your mother can eventually gain citizenship. She can apply when she turns 55 because then she will not need to pass the test any more.