Wills are documents with specific requirements for how they are written, signed, and witnessed, and each state has their own variation on those requirements. A separate statement accomplishes nothing but further muddying the water.
Your father should see an estate planning attorney and create his own estate plan (will or trust and supporting documents). There are 173 on this website for Houston alone.
Whether or not your father signed the will your sister prepared isn't a big issue at this time, since your father is still living, and can simply have another will prepared. Preparing a new will would be the best solution to the problem. Contact a lawyer in your area for help with this, and have them look at that trust agreement, too. You can call the state or local bar association for a referral if you don't know a lawyer.
The answers by the other contributors are correct. If your sole concern is your father's will, all he needs to do is prepare a will, stating that it revokes all prior wills. If, however, there is a concern with your mother's estate, it may be necessary to contest the will which your sister prepared if you believe it to be forged.