My father passed away Thanksgiving 2018. When is it required by law that the executor provide a formal accounting of the trust ?

If there's a trust, your sister is the successor trustee, not the executor. She should not be living in the home for free, unless she is given the home in the trust. If the trust doesn't give her the home, she should probably be paying rent, and working to get the house on the market as quickly as possible. You can send her a letter requesting an accounting, and asking when she will put the house up for sale. If that doesn't produce results, you probably need to hire an attorney.