Is 5'4 an ok height? With 120 lbs? Description!?

Well, my BMI is 20.6. I think this is pretty good. My body structure is kinda meh because most of the fat is stored on my buttocks and thighs. It's not like I have humongous thighs but they are a little fuller. And I just turned 15. My grandma always compares me to my 18 year old cousin which is REALLY thin. I think she's too think. Her jeans don't fit me. My grandma said that I am heavy machinery compared to her. I felt pretty bad. I use uniform for school and my grandma makes my skirts. She does them so tight around the waist I feels like a corset. I think she is doing it on purpose or I don't know. She makes me think that there is something wrong with my body. I have already talked with my mom about this and she says that she thinks I am perfect the way I am since I have meat on my bones and I have an hourglass body and not a ruler. So yeah----