The parent in question estranged himself from his children and did not include them in his will, instead including his girlfriend. Upon his passing, what are the rights of the children, now as adults? Is there an assumption in California that the intention of the deceased as stated in the will has priority over the rights of the surviving children?
Added after Mr. Kenneth Lewis Swenson's reply:
Is this regardless of circumstance? The estrangement was due to the father's actions while the children were young, and continued despite efforts by one of the children to make contact as an adult. The father's actions allowed him access to a very successful career, while the mother worked at entry level jobs and the children later had no financial support for a college education. No alimony or child support was ever paid, despite court order.
Yes, valid will controls the disposition of the estate. Excluded heirs may challenge the will under certain circumstances. If the estate is significant, you may wish to consult with an attorney to discuss the particulars of your matter and for specific legal advice.
The additional information you provided does not change the initial answer. Your mother could have sought relief during your father's lifetime for money owed to her under the court order. If she is still living, your mother may make a claim against the estate for unpaid child support.