Why do most older siblings have to be mean to their little siblings?

I mean come on, even if their 3-7 years old people are so mean. My younger cousin who is 12 is mean to her 6 year old sister but not abusive way. She always so hard on her and I told her she is just a little and go soft on her. Of course she didn't listen to me and said that just because she is little doesn't mean people need to be nicer to soften up. That was ever so harsh. My older siblings were very touggt on me because they know I'm very sensitive, especially my older sister who is blunt and straightfoward since I can be naive at times. She told me to stop getting upset and get sensitive when someone is honesty and brutally honest with me because they're will be people like that in the world. She loves me and I love her but she is so tought with me. Why are older siblings so "mean"? Being mean is never a good thing. We should be kind and loving to one another.