My first impression was e-mail sent to the wrong parent (but no, it calls my sister by first name) as there are actually 3 players with nieces name, or my sister had made it up as a joke because I kind of laughed at first it seemed so bad. Both not true, sent from the school. And it was still not professional if sent to any other girl.
“Then why e-mail her mother saying the daughter sucks?” My guess, only getting one side of the story, maybe the coach is sick of getting grief from your sister, you did say that she had a big mouth.
How do you know for sure that the couch actually sent it? Maybe someone used his computer???