I understand how things must feel when it come to family, but your sister-in-law must understand that this is your home and their are some rule she must follow when it comes to your home, there are certain tools people use as an excuse so they can have a reason why they don't do something, this seem like the case with your sister-in-law who seem to use your son as her reason why she don't clean up and which is not an excuse for her actions.
We all know how hard it is to even get teens to clean up their own rooms and I know it must be hard also with the pressure of not only your teen but your sister-in-law but this give her no excuse for her reason as an adult to compare her self to a child, your husband must begin to understand your point and address this issue towards his sister or other action need to take places. You and your husband need to set a date and time to speak with your sister-in-law about this problem and when is she planing to finding a place of her own.