Firstly, If fmly visit visa issud by Main branch of Immigration and passport control authorities than its extendable upto six months provided within one month of arrival yr mother in law need to go for a medical test at Medical Commission.And u have to buy a return ticket showing return booking date after one month.later if visa extend u can extend return travel date with nominal change reservation fee.Return tkt is mandatory for all visitor.otherwise airlines representative will not allow her to travel.
Secondly,if yr visa issued by Doha airport immigrantion office than it will be valid for one month and its not extendable.after one month u have to leave Qatar.hope it will help yr query