Rankling as it may feel, your sister and step-sister's actions and inactions may well be perfectly legal. But you describe an estate plan that seems to involve a lot of
different documents, powers, and people named to control, so getting to the bottom of who controls what may take some patience and sleuthwork.
The first thing to keep in mind is that the power granted by a power of attorney dies along with the person who appointed that agent, so it is likely that your older sister is not responsible for any of the current financial management. In your scenario, that may help straighten out some of the mystique, but perhaps not all of it.
The key is to find out who was appointed trustee of the trust that currently exists. That is the person who has the right to control and manage any remaining trust assets. Usually the trustee is obligated to act in the best interests of the trust beneficiaries"”although trusts can also contain specific directions about spending assets or about providing for the care and maintenance of another person. So if your father crafted the trust to provide for his wife's care, your step-sister may simply be following legal orders if she has been named sole trustee.
One beauty and bane of trusts is that their terms are private, unlike wills and many other types of estate planning documents, so it can be difficult for people"”even surviving family members"”to discover their terms. From a practical standpoint, your best bet may be to approach your step-sister informally and noncombatively, simply explaining that you're interested in some of the specifics of the trust management, just in the hopes of avoiding bad blood down the road. In a surprising number of cases, this can clear the air.
But if this approach isn't possible or isn't successful, your last resort may be to petition the court in your step-sister's state, demanding an accounting of the trust administration. Courts usually have the discretion to grant or deny should a request"”and some will stipulate that an impartial third party must review the accounting.
But even demanding this accounting is something you should weigh carefully. Unless you have some fairly good evidence that there has been some misconduct, the action may add to the strain of your relationships. As a last resort, however, it could simply underscore to your step-sister that she needs to act honestly and fairly and as the trust directs.