four year old mimicking bad behaviour.

My four year old daugter and I currently live in a blended household which consists of myself, bub, my partners mother and her two daughters eight and fourteen. Trouble is her eight year old is a very naughty very rowdy and quite frankly overbearing whiney annoying child and all her bad behaviour and habits are rubbing off on my dwughter as she was nearly two when we moved in and copied her every move. I just want to know if its possible to reverse this behaviour as I cant bear it any longer I feel like I haven't given her the opportunity to figure out her identity because she has been suppressed by the louder personality and followed suit. This child also displays some symtoms of ocd which I cannot have rub off on my baby. I thought about leaving it until she was older and explaining to her that its not the right way to behave, like hissing at someone when they try to stir her up or make a joke of something she did. Violent play and stupid noises are also on the list. I am feeling like a terrible mother over this issue help!? What do I do??