The only thing horrible, cold, and senseless is the behavior of the sister-SO FAR.
Not saying you are right or wrong regarding the situation but we are not there and don't know the whole story. Sounds to me like there is more going on than what has been reported cause I can't understand either how if you love someone and they are family memembers especially a sibling that you would not step up to the plate and help them out. No matter how you slice it though,those kinds of negative remarks are unnecessary and don't contribute a thing.