No wonder you are feeling lousy ... the environment and rules have changed and it sounds unclear what they actually are and they keep changing. The nepotism seems especially intolerable. Maybe you could ask for a written job description if you don't already have one so that it is clear to everyone what the expectations are. Sit down with it in hand and ask yourself if this is the environment you really want to spend your time in. If not, start looking for a new job. A friend of mine was recently in a similar situation and she bumped along for a while but recently found a new job (less pay) but she is so much happier.

(((Hugs))) Work can cause such stress ... but a good work situation can be very fulfilling. Be a good employee wherever your work but take care of yourself, above all.