I totally know what you are talking about. I posted on here about a month ago because my babies were fighting so much and I was afraid they were going to hurt each other. My two are brother and sister, but my girl is literally twice the size of my boy. She is obviously the dominant one in our house. She is always biting him and attacking him for no reason at all (just for the heck of it I think). He'll be laying on the floor chewing a toy and she will just walk by and bite the heck out of him and have him yelping.
I let it go on as long as I think they are playing, but when he starts crying or yelping I usually "rescue" him and he will shake for a minute or two in my arms. Poor little guy.
His sister is such a bully. If you figure out what to do, let me know. I could really use suggestions too.