How do I deal with an annoying stepmom and her daughter?

I'm 16 years old and 11 years older than my sister. I cannot stand her!!! She always says that she misses me sooooo much when I come back from my mom's house, but her actions say otherwise. I can't help but being extremely bitter and paranoid when she's around and everytime I tell her to stop (let's say bouncing a ball in the house), I GET IN TROUBLE because "I need to act more like a sister.... not a mother." She has pushed me to my limit and I have gone and cried because it is tearing me apart!!! I feel like my stepmom ALWAYS tries to protect her from getting in trouble... I've concluded that that is the reason why my sister is such a brat. Help please...