I'm a joint account holder with my dad and my sibling is paying their own bills out of the account currently with parents consent. Can I stop these payments if dad becomes incapacitated?

If your dad has enough money (7 figures) to afford several years of Nursing Home care without worrying about utilizing Medicaid, then he can allow this to continue.

Medicaid will review 5 years of CC and bank statements. Any monies that were gifted to others will result in a disqualification penalty, meaning someone will need to care for him at home.

Make sure he understands that, and that your sister realizes that she will be the one doing the work.

You point out that you father can make his own decisions. Talk to him about the situation. Not in an accusatory manner, but out of love and curiosity. Ask him if he realizes what your sib is doing and what his take on it is. If he is okay with it, then proceed as normal. If he is not okay with it, talk with him about how he would like proceed. Does he have a POA for his finances? If he can make his own decisions, not is the time for his wishes to be legally documented. Make sure to have your sibling(s) involved in the conversation about how his money is being spent now if possible. This will help clear up any confusion in the future. It is his money and knowing his thoughts now, in case he ever does become incapable of making decisions, is important for you and your sibs.

Your dad needs to see an eldercare attorney pronto to clear up this mess.