KMC Forums - help i need to know this movies name ???

help i need this movie's name ???? the i think it's kinda old maybe in the 70's and 80's any way it's about a family who've just bought an old house which is possesd by a devil demon whom later will posses the older boy in the house the family are a mom ,dad one older boy and an older sister and i think 2 lil' kids it's all started when the mother went to the basement and tried to open the sink but it fresh blood came out insted of water , anyway all i remember is the boy when he got possesed by the demon after a while he was looking @ his sister getting naked through the room's key hole and then he slammed the door open and then he slpt with her the mother got suspisiouc from the older boy and desided to get a priest to bless the house and take a look @ the boy's condition but the boy saw the priest he screamed in a demonic voice and he was floating in the air and then fell down so the priest took the boy to the hospital and while he was sleeping blood drops starts to come out from his left wrist shaping into words saying " please save me " so the priest desided to save the boy and kill the demon so while they we're fighting and struggling the priest saved the boy but the priest didn't make it and dies in the sudden fire that started in the house burning it 'till the next morning when the house was all black and i think that's it ,so please if anyone who knows the name of this movie i'll be grateful ,, thx a lot guys