Why is the AIP obligated to pay the legal bills of the person who has tried to be named legal guardian?

I am the POA and for both my father & mother. My sister sued for guardianship for my father by alleging that I have been "misappropriating" his money. Which was not true, during the hearing, it was not determined that he was in capacitated and that he totally trusted me to handle his financial affairs, which I have been doing for over 20 years. However, the court determined him to pay for not only my sister's expensive lawyer, but to pay for the other court appointed lawyers as well. Why should my dad, who is on a fixed income, now have to pay for this frivolous legal action? This action was nothing more than an attempt of a money grab? I have been taking care of my parents who I live with. What are my legal options? Can I sue her for defamation?