During my parents marriage my dad had an affair that lasted some years. It caused my mum huge amounts of distress and ultimately after suffering a nervous breakdown she died by suicide. My mum had her demons but she was a truly amazing and selfless mother and her death was extremely traumatic. My siblings and I were in our early 20s. My mum told us and I was made aware by other family members that this woman tormented my mum at her lowest ebb. My dad never actually left my mum and refers to himself as a widow and wishes to be buried with our mum when the time comes (complicated I know). Our family has over some years recovered and we have a decent relationship with our dad of which I am proud and hold dear as despite what happened, he's our dad and we love him. Some 15 years after my mum's death (and with 3 significant relationships in between with nice women) my dad has now started seeing this woman again and she has been thrust into our lives without warning. I find this very difficult to deal with and try and see my dad separately to avoid distress but this is becoming increasingly difficult as she comes to all family occasions and introduces herself as my dad's "partner". She is very pushy and not at all likeable. For example, she reminisces about weekends away that her and my dad have been on years ago when he was married to my mum and we were picking up the pieces and seems to have no remorse or regret about the circumstances. I am expecting my first child and whilst I, for the sake of my relationship with my dad, am willing to be civil to her, I cannot stomach the thought of her seeing or having anything to do with my baby. I feel like this would be a betrayal of my lovely mum who I miss greatly whom I would have dearly loved to have been a grandmother. I want my dad to be happy but I cannot let this go. AIBU?