She is constantly looking down on my mother and since my dad is so hot headed he looses control on his anger and end up taking my grandmothers side as well.I feel like saying very hurtful and pricking things to her to get her mind straight but is it ok for me to act this way?
I'll also bet your grandma is your dads' mother.
Your dad is probably upset about his moms dementia (I'm supposing she has it). Grandma isn't in her right mind and is picking on your mother possibly because she can see her own mental decline and that your mom is mentally together, or she knows your dad, (her son), will take her side and that gives her some satisfaction.
Your mother needs to set boundaries and limitations with your grandma on how she is treated. She needs to discuss this new approach with her husband (your dad) and get his cooperation and understanding. He is between two very important women in his life and he should NOT take sides. If he's a hot head then he should leave the room during an altercation.
If the situation can't be remedied, then grandma will have to go somewhere else, (another family member or nursing home). One person can not and should not break up a family (Your mom, dad and you).
You should NOT say mean or pricking things to grandma. I know you want to "set her straight" but that's not possible with dementia. She won't "get it".
I'm sure you want to defend your mom also, since she's the one being picked on, but she is an adult and should be able to survive the verbal assaults. She doesn't need you to "pour more fuel on the fire". If your mom keeps calm and sticks to her boundaries, then grandma won't get the reaction she's used to and (possibly) wants. Hopefully, it will be enough to turn your situation around and she'll give up.