Who is the legal heir to the estate?

There is an absolute difference between being a trustee and being a beneficiary. If your sister was added to their bank accounts and their deed, not as a trustee, but in her individual name, then legally those things belonged to her after the death of both of your parents, and are now part of her estate, not your parents' estate.  However, it is very likely that is not what your parents intended. If she was added only for convenience, you have a legal right to regain half the estate. Your niece, as your sister's heir, would be entitled to the other half.  If you want to pursue this, you will need to hire a local attorney. This is not a minor legal battle, and will incur costs in the thousands of dollars.

If there is no Will you and any children of a child who predeceased your parents may be the heirs. But Wills and heirship only govern property which is not titled in the name of a trust.  Some trusts do provide for the successor trustee to name her own successors.  In any event, the trustee or successor trustee must follow the terms of the trust in making distributions and, in most states, provide an annual accounting to any beneficiary over a certain age.  Send a letter certified mail, return receipt requested, demanding an accounting.  If you do not receive one within 60 days, hire a lawyer.  You may not be a beneficiary.  Or you may be a beneficiary and need to have the trustee replaced by the court.