If my brother has my dad in hospice in my dad’s house and wont allow me and my children in to visit my father, what can I do?

If your father is mentally competent, you can't do anything (unless you feel your brother is abusing, basically "holding hostage" or confining [not letting your father have visitors *when he--your father--asks for them*, or not passing along messages, etc.], or stealing from your father, in which case you can report the situation to the authorities: e.g. to adult protective services and the police; but you have to believe he is committing a crime to do this). A competent adult can decide who to see and not see, however, so if your father is letting your brother make decisons for him and is willing to not see you or your children, that is his right. You an only intervene if your father is being deprived of his liberty, property, rights, etc. (e.g. again, essentially criminal acts) or if he is not competent (see below).
If you feel your father is not mentally competent at present, such as due to his medical condition or medication, then he can't decide what to do and who to see and needs a legal guardian; you can file a lawsuit in family court on an "emergent" basis (think "urgent" or "emergency" basis) to get into court more quickly and have a legal guardian appointed. (You can request that the guardian be you; the court does not have to honor that request, but will consider the evidence of who would be best for the role.) The guardian will then decide access to your father, based on his best interests.