I'm going to take my sister with me to my grandparents house without telling my mom
Why do you need to keep that to your mom?
I have a better chance of going to grandma's house without my parents stopping me from going there.
Your sister is your mother's daughter. She would be very worried about both of you if you were just gone without telling her. In the worst case, this could lead to law enforcement looking for lost children and make trouble for everyone.
Calm down, I wont have that problem since my parents trust me.
Even if your parents trust you, they do not know where you guys are going and they would still be worried about it. I do not think it is healthy to have them worried unnecessarily and there may be a reason why they do not want you to go in the first place, so try and respect and consider that aspect as well.