Jenni - you can talk about your feelings here. From only reading your original post, I didn't get an idea of what kind of person the mother was. After reading your follow-ups, I think you might be off-put by the character of the mother, who appears to be two-faced. Getting really close to you and then excluding you.

Were you able to talk to her yet? A lot might be cleared up if you find out whether or not she had been purposely trying to hide this event from you (and if yes, why she did it).