Dad is widowed has a girl friend that he left as sole beneficiary did not mention his kids in the will what can we do if anything

In Texas people can leave their things to whomever they wish.  You might consider contesting the Will if you have evidence tending to prove fraud, lack of legal capacity or undue influence.  Your description indicates none of these.  The last two can be very difficult to prove.  A Will contest can cost as much or more than you might inherit with no guaranty that you will win.