You can sponsor her almost as soon as you land but you must be able to show income (job contract? or first couple of payslips?)
Your wife would have to sponsor her because it's her mom but you can co-sign so both your incomes count.
http://www.cic.gc.ca/english/information/applications/guides/5196E10.asp will show you how much money you need to make, go to table 4 and add up your family members including mother in law.
I think you should have a pretty good chance of success. It takes a long time to sponsor a parent though.