Hello TheAntiBody,
Could you please tell us which of the following applies?
A) Your mother has already become a permanent resident (called "landing").
B) Your mother has already received the document that will allow her to become a permanent resident (probably a "confirmation of permanent residence" or COPR), but is not already a permanent resident.
C) Your mother has begun the process to become a permanent resident, but has not yet received approval. CIC has not yet issued a COPR.
I think that in case A, it will probably be impossible for you to be sponsored by your mother or her husband in the future.
In cases B and C, I think it is still possible, but you will need to hurry. Your mother and her husband would need to submit an application for you before you turn 22, and before the end of 2013.
I think that in case B, your mother will probably need to return her documents to CIC and start a new application for her and you, but Scylla said this might not be true. This will delay her becoming a permanent resident. In case B, if your mother and her husband want to help you immigrate to Canada, She should NOT become a permanent resident until she has received legal advice.
In case C, she may be able to modify her current application, but you will need to check with CIC whether this is possible. This will probably be much easier if she does it before CIC issues her COPR. In case C, she should contact CIC immediately to tell them she wants to make you an "accompanying family member".
If you are close to your 22nd birthday, you really need to hurry.
Edit:
It's not very clear from what you've written whether your mother and her husband want to help you immigrate. You can only immigrate to Canada through them if they apply to help you immigrate.
First, they may need to pay new application fees. Also, your mother's husband will need to agree to "sponsor" you until you turn 25. This means that if you go on welfare before you turn 25, he will need to pay that money back to the provincial government. (Welfare is money that the government pays you if you are very poor.) So if they are convinced you won't be on welfare for the next three or four years, this should not be a problem.