I got my mother-in-law here in Qatar last year when my wife was pregnant. There were no issues at all, and we got her visa in 3 working days after applying for the visa.

You need following documents along with visa application form:

1) Copy of your Passport with Qatar Residence Visa
2) Copy of Passport of mother-in-law
3) Copy of your Marriage Certificate
4) Copy of wife's Birth Certificate
5) Letter from Sponsor citing profession and salary

Hope it helps!