Can I contest the actions of a trustee selling a house that is in trust?

Your first resource must be the instrument by which the trust was created and any amendments to it.  It will specify whether the three trustees must act unanimously or if "majority rule" prevails.  If the trust instrument does not specify, California law provides that the trustees must act unanimously, and I assume your "little sister" can stop bad things from happening.  If the trustees must act unanimously, your little sister will need to sign documents in order for th house to be sold. If the trustees may act by majority rule, your little sister has the right to know what the other two are doing and to report that to the beneficiaries (including you).  That right can, if necessary, be enforced by a court.  The trustees are required to invest trust assets prudently and to follow the terms of the trust.  The trustees are bound by fiduciary duties to the beneficiaries (including you). The trustees ar required to report and account to the beneficiaries.  Assuming the trustees are not doing these things, you have a right to seek relief in a court.