It's particularly distasteful to take advantage of a situation in which one has a dying close relative to sneak around behind the back of another close family member.
Years from now you're going to be kicking yourself for spending the time you could have spent with your Grandma, trying to grow weed in the house since your mom's not there...
Priorities, son. You've got all the time in the world to grow pot. But grandparents don't live forever. This might not be the best possible moment to be busying yourself with what amounts to a distraction at this point.