Question on contra UPCI apologetics?

My cousin, who I used to be very close to, has been converted to some groups called UPCI -apostolic oneness pentecostal. He told me I am going to hell because I believe in the Holy Trinity doctrine and because I have never spoken in tongues. They say that Jesus is God and the Father and the HS are manifestations of Jesus. On the face of it, it seems silly that Jesus was praying to himself when he sweat blood in the Garden of Gethsemene . But my cuz has been brainwashed by some pastor who has spent his whole life trying to make these arguments. I went to catholic schools all the way thru and had some Bible and catholic apologetics, but I have not studied the stuff myself in many years. Can you help me?