Can an heir who is the trustee of his sister's special needs trust sue her trust to get money for another sibling who was written out of the will?

No. The heir has no right to the funds set aside in the special needs trust. A living donor set that trust up with their own property and the heir has no rights to that property whatsoever. The trust is not responsible in any way for a sibling who was disinherited. Parents are allowed to disinherit their adult children.