Wills, Trusts, and Estates
If a trustee undertakes a transaction that involves a conflict of interest between his fiduciary capacity and personal interests, courts will find that to be a violation of his fiduciary duty and make no further inquiry.
The only defenses are that the settlor authorized the conflict in the terms of the trust, the beneficiaries consented after full disclosure, or that the trustee obtained judicial approval beforehand. R3T § 78.
The no-further-inquiry rule does not apply to structural conflicts created by the settlor. E.g., a trustee on a company's board is given shares in that company. He can vote for himself still if it is in the best interests of the beneficiaries.