hi,
you can extend the visa of your mother once you got it for 6 months, every extend will cost you around 200 Qr, after the six months expires, she must leave the country for 3 months before she can come back again, and even if you applied before the 3 months expires, the application will be rejected, this applies to all family visas, mothers, sisters, wifes (for some nationalities this is the only way a man can get his wife due to nationality quota issues).