My siblings changed my father's documents without notice. Where do we go from here?

My father has been ill for many years with a heart condition. He also was married several times and was having some difficulty with his current spouse. I went with the two of them the first time to draw up their will. It is important to be honest and fair even if you do not care for someone and I think that is why my father had me go with. I notified my four other siblings and let them know what went on and sent them all copies. He wanted me to be POA and executer of his will. I told him I would but my eldest sister had to be on with me because I thought that would be honest and fair. My father had his will changed and asked 3 of us to come along. the other two were out of state. He again had it updated with my sister and I as his POS and Executers. I spent a lot of time with my father and he spent a lot of time at my home. The past two years things changed. I should have realized that something was wrong. I probably only talked to him 5 times in the past 2 years. He seemed like he did not want to talk to me anymore. He passed away 2 weekes ago. When he was sick we were not told what hospital he was in and when he went to a nursing facility we were again not told. When he passed that morning we were not told until 8 pm that night. All the communication I had was through my younger sister. We found out later that anything my name was on they had changed. The will, safe deposit box, POA and insurance policy. His small house that he owned he had me change quite a few years ago and we put all 5 childrens names on it as owners. My problem now is who do I contact to get this mess straightened out. I don't care about all the money they took but the house needs to be dissolved and equal portions to each remaining child. They do not talk to me or my younger sister any more. When she asked about the will all the eldest sister said is that they changed it and they are in charge of everything. Where do we go from here? Who do we contact to get this resolved.