If you want you to compare your grandma to the real world you should take something you belive no one eles can do and compare it to something else. Instead of making it equal you want your grandmother to have the good review and topic you are comparing to should have the negative.
How can I use a simile to compare the real world and my grandma?
Here are some examples:
"When my grandma is determined to show off her skills, she would act like she was competitor who was showing their talent to a judge."
"My grandmother always knows when I try to eat a snack before dinner, she is like an eagle watching it's prey"
(Hopefully this is what you're looking for! : )