She feel's her dad's will betrays her son

Regardless of your father's thinking when he drew up his will, you should not pressure your mother to change it.

You are in a position to either make this worse or to make it acceptable for your child's sake. By acceptable, I mean, "something you must accept." You are not betraying your son by having a relationship with your mother. The message you should convey is that family members have flaws, but we are each capable of having a relationship with someone despite these flaws.

Say, "I disagree with this choice, but I cannot make it right." You could mitigate this somewhat by splitting or sharing your inheritance with your son when the time comes.