Could you call some volunteer groups to come and give you reprieve from time spent with your grandmother? Or if she does not have any issues getting around perhaps someone could take your grandmother out of the house for a little bit. You may contact Hospice first to see if they have any suggestions for groups who can help you.
I commend you for stepping up to the plate and taking care of your grandmother in order for your mom/dad to have a much needed break.
Try to remember when your grandmother was in better health, a sweet, loving, caring person you enjoyed visiting. Many times strokes will cause brain damage and the person will no longer be the way they were before. It is not their fault their brain messes up with their personality, etc.
If you can overlook many things, then there will be no need for temper. We should not get angry with anyone who no longer has self control.
I personally know how hard it is to care for a person after heart attacks and strokes, it is not easy.
Remaining pleasant and doing the best you can will make life more pleasant for you and your grandmother.