Should I tell my mom my sister was speeding?

My sister just turned 16, got liscines, started driving alone. She told me that on our street, where the speed limit is 30, she went 95. Then she told me SHH don't tell. But I feel like I should tell, cause that is dangerous. She has already speed with me in the car ( only like 10 over) what would you do? I'm 14