Alice, My Mom Refuses to Attend Events If My Dad's Girlfriend Will Be There

Dear Alice, My parents have been divorced for nearly 20 years, and my dad is now remarried to a lovely woman. My mom refuses to interact with my dad and his wife, making for very uncomfortable/stressful moments when we are forced to all be in the same room (graduations, weddings, etc.). A few of my mom's friends are throwing me my baby shower in a couple of months. My dad recently ran into one of the shower hosts and asked her to include his wife. I don't want to hurt my dad's wife's feelings, b...