Why is your mother being loyal to your ex husband and not her daughter?

A quick assessment might be that your mother is manipulative. Unfortunately, you don't have enough info here to give a useful answer. I recommend that you sit down with your mother and discuss with her why she feels this way and how you feel about it. If she isn't willing to discuss it, you might take it to a professional. Remember one thing, you can't change other people's behavior, you can only change your reaction to it.