Make sure all food scraps are constantly cleaned up, invest in roach poisons, get some gas bombs.
Put the roach poison in spots that roaches hide (mostly in the dark spots), without any food they'll die, finish it up by placing 1-2 gas bombs in every room and setting them off (then run like hell out of the house before the room fills up) make sure you and your Grandma are gone for a few hours. If this doesn't work than try gassing it again in a month or so. Keep laying out the poison and insuring the place is clean.
You don't have to call an exterminator over this, they'll just overcharge you.
