Do not waste your time in proving to your family that Satan is Truth. They are and will be responsible for their own action,beliefs,whatsoever as everyone is. They will NEVER believe you because parents really tend to think that they are kings as Zevios said & they know everything and whatever they know is truth. Rather put your time into an online spiritual warfare to spread the truth.
Just Giving some advice here. Some people will go and tell others that their god or religion is false,their god is monster,or whatever and Satan is the only true god. For sure they will never believe on you because you attack on their false god/religion while supporting Satan whom they know as an evil being. The debates will go on and on and turn into a mild fight. What i found the correct method to make them believe is use ATTRACTION, To *ATTRACT* them towards you. "Tell something little about Satanism or if not satanism then something extra-ordinary that may attract them to you,for this they will be willing to know more about the half little thing you've told them. Instead of shouting and spamming JoS link, Be calm,wise,speak little,make the person to be WILLING to talk to you / to know more. Just show little POSSESION of Truth. And to make someone/family believe by performing telekinesis or something extra-ordinary then think twice before doing it. Because they will Freak out and get scared of you and the words that will come outta their mouth because of lack knowledge & ignorance, would be like : "OH MY GOD,YOU ARE NOT OUR SON,AN EVIL SPIRIT OR GHOST IS WITHIN YOU THAT IS DOING THIS ALL STUFF" ! Lmao! And then they carry you to the church.

phew !