Legal Question in Wills and Trusts in California

Is it legal to do online banking with a trust account? If the bank allows you access online to a trust account, and the trust document does not prevent it, than it should be allowed.

Can you close the account with the others knowledge or signature? If the trust document says the co-trustees are to act together, then no. If the trust document says that each co-trustee can act independently, then no, the act of closing the act would not be per se "illegal".

As a trustee of the trust, each of you has a fiduciary duty to the beneficiaries of the trust. If one trustee takes money from a bank account and uses it for their own needs, that act may be a violation of their fiduciary duty.

There is no prohibition against doing online banking with an account held within a trust.

However, if you and your half-sister are cotrustees and she acts to remove money from the trust accounts in violation of the terms of the trust, then she may be in a breach of the fiduciary duty owed by trustees to the trust beneficiaries. You should determine the amount of the money your half-sister removed from the trust and figure out the loss to the beneficiaries. Then you may want to review the situation with a trusts and estates attorney to see if it makes sense to take legal action against your half-sister to get it back and perhaps take other actions such as removing her as a cotrustee of the trust.