can children and/or siblings step in to decide where their relative (father and brother) should be receiving rehabilitation after an aneurysm/stroke

He himself has the choice.  If he has named an agent under a Medical Power of Attorney, that person can be his voice.  If not, most states turn first to the spouse.  Frustrating though you may find the current situation, you do not have authority to alter it.  What you can do is try to persuade the wife, perhaps in a family meeting, perhaps in a family meeting mediated by a social worker or professional guardian experienced in such matters.  Ask the social worker at the facility for recommendations or contact your state guardianship association.  Be sure to gather as many facts as you can first.  You may think that he is not being provided the necessary therapy, but the doctor may have ordered what he or she finds appropriate under the circumstances.  Your first visit may be with the doctor.