Is your mom a PR or citizen?
So she applied for your ex's PR, under
your name, and forging
your signature multiple times? This is a year long process typically and you didn't realize until now?
You need to report this to IRCC/CBSA
immediately. That is the only way to prevent this from happening. You can simply try to cancel your sponsorship but that may not be enough if she already has COPR and will land soon.
http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=732&top=14
If you don't report this, then
YOU are also committing fraud by allowing it to happen and doing nothing. If IRCC finds out about it later (which they probably will if you try to sponsor a new spouse anytime in the future) then you could lose your PR status over it.