There very well may have been abuse prior to this but even so one cannot necessarily blame the mother for it with a child that age as (like someone else pointed out) they're constantly getting hurt one way or another since they haven't developed really good motor skills yet nor do they have any of what we would call "common sense". The babysitter could have made up excuses about almost any kind of injury and still be in the believable range. Not really so much with an older child. That's why it is so important to make sure you only leave ones so young with someone you really know and trust like a professional or a close relative (grandparents are always the best and studies have even shown children are slightly safer with them than they are their own parents).
In all seriousness I don't really see why so much focus is on the mother, anyways. No matter how young the mostly likely instinct of a mother is to protect her child from being hurt by others. Usually if a mother is whacko she takes the child out herself instead of conspiring to have others take the child out for her.