First of all, your aunt is not and cannot be your father's legal guardian unless and until she is appointed by a probate court judge. I suspect that what he signed was probably a Power of Attorney form. That form is designed to give someone the same power and authority as a guardian, without the need for court proceedings. Generally, that is the preferred way to accomplish what you are trying to do. There is still a possibility that your father could redo his documents, depending on what his level of understanding is.
I would suggest that you contact an attorney and have him meet with your father to determine whether or not it is possible to amend his documents at this time, without going to court.
If you determine that your father does not have sufficient capacity, then I believe your last resort would be to go to court to have someone else appointed guardian. This will undoubtedly be more expensive than redoing the estate planning documents. The other downside is that once you are in court, you are not in control of what happens, or what the judge decides. You are dealing with someone who does not know you or your father, and does not know what he would want. It is possible, (although I think it pretty unlikely), that the court would appoint the sister, over the wife or one of the children.
The other thing that your MOTHER should do is make sure that all of HER estate planning is in order. Obviously, she would not want your father or his sister involved in any of her documents. It is possible, if not likely, that some if not most of the assets are titled jointly between your father and mother. If that is the case, his estate plan will not affect the assets, or your mother, all that much. In any event, it is a really good idea to have a qualified estate planning attorney review what your parents have and try to determine how best to proceed. This should obviously be done as soon as possible.
Best of luck to you!