My sister is trustee of my parents trust, she entered into a personal care agreement with another sister after parent died. Any advice?

Trustee entered in to said agreement on the sole word of person to give care (also a sister and beneficiary of trust). Agreements states that the care giver will be compensated for current care given to surviving parent but also compensation for care given for past supposed care. Statements in contract can be proven wrong in regards to things in past like taking dad to cancer treatments, care of father when hospice was actually doing care. Caregiver (sister) said there was an oral contract with father which no one else was informed of in 2012 yet she only put agreement in writing after father died and my mother was said to be mentally unable to make these decisions in which the successor trustee takes over. This happened in 2016 and no one knows who had our mother declared mentally unfit. The gist of all this is ...due to this contract my sister ( also said caregiver) has laid claim to almost entire trust, and trustee may have been acting genuinely in the interest of parents but has no actual proof of anything agreed to by either parent. Understand contract was only put in writing after both parents were either passed on or apparently said to be no longer mentally capable of being trustee. The amount personal care agreement states for compensation is in the hundreds of thousands. We need help in figuring out how this came about, why if agreement WAS made with parents in 2012 why only put in writing 4 1/2 years after one parent passes and several days after sister (successor trustee) became trustee after supposed declaration of mother's diminished mental state.