This sounds like a tough and delicate situation. The first thing to understand, though, is that if there is a valid power of attorney and the person with that authority
is acting legally, you do not have the legal right to override those decisions simply because you are the adult child.
There are several things to consider regarding your efforts to do the best thing for your father. One of the first things to get clear is the extent of authority granted to your step-uncle. That means determining whether he has been granted a general Power of Attorney or Power of Attorney for Finances, which gives broad power to make decisions about your father's life, or only a Power of Attorney for Health Care, which only allows your step-father to make specific health care decisions. If the authority under the power of attorney is limited, by the terms of the document itself, that may leave you room to act on your father's behalf in matters that are not covered. Take a look at the document itself, and if you're not sure what the authority is that the document grants, take a copy to a lawyer to get some legal advice.
The next question is whether your father is still legally competent to make decisions on his own behalf. In other words, is your step-uncle making decisions under the power of attorney because your father is not physically able to handle his own affairs, or because your father is also no longer legally -- meaning mentally -- competent to make decisions? If your father is still legally competent, he can override your step-uncle's decisions by simply telling your step-uncle -- in the presence of others -- that he wants a different choice than what your step-uncle has decided. If you let your father know your feelings about decisions your step-uncle is making that you don't agree with, and you explain other, better options to your father, maybe your father can override your step-uncle in this simple way.
If your father is still legally competent to make decisions but directly overriding your step-uncle's decision seems too confrontational or difficult for your father to do, your father instead can revoke the power of attorney he previously gave to your step-uncle and execute a new one giving you or someone else power of attorney authority. Given the delicate family circumstances you're in, you should have several competent people witness such a new power of attorney in case your step-uncle contests it.
If your father is no longer legally competent to make decisions for himself, things are more difficult but not impossible for you. If you believe that your uncle's decisions are detrimental to your father's health or well-being, or if you believe that your step-father is in some way abusing his power of attorney authority (such as by using funds improperly, or by favoring his mother over your father in the use of family funds), you can ask a court to intervene by making your or some other family member your father's adult guardian or conservator. These court proceedings can be difficult, hotly contested (for example, by your step-uncle or other family members), and expensive (almost always requiring the help of a lawyer). But if your father is no longer competent to make his own decisions and you believe he is and will continue to be suffering because of your step-uncle's decisions, this may be the avenue you need to consider. If you do, you should get in touch with an attorney who specializes in adult guardianships and conservatorships to find out what the procedure is in your state and what the likelihood is that you would succeed if you went to court.