Infidelity Support Group
Any relationship in which one partner engages willfully in sexual relations with another outside of the partnership is considered to have experienced infidelity. This breach of trust is often traumatizing for the faithful partner as well as the relationship, and support is often needed to heal emotionally and to decide whether or not the relationship should continue after...
My suggestion would be to tell both your parents that you are having a difficult time with this.
All of you should be going to family counseling. Things are changing and they are changing
quickly.
Rather than talk to the other woman, I would say write her a letter. You could then give the letter to the counselor who could advise you on how to best proceed.
Talking to her might not be the best thing for you. This is a woman who got involved with your father knowing he was married and had a family. Then she got pregnant. She doesn't give a hoot about you. I'd stay away from her.
Good luck to you.