How about putting the brother & sister together in the same story with divinity? The story could go like this...

Standard version:

Dark divinity wants to break/go back into the world of the living. He finds this child that was borne with the abilities that he has to have in order to succeed in his attempts. So one night when boy was playing with his sister divinity pulls him into his domain. Sister is in shock and also no one believes in her story so she decides to get him back herself.
Brother manages to create mind connection with her and instructs her to go to the old ruins close to the edge of the town. There she enters through the ancient well that is portal to the lands of the pagan god.
They both has to conquer the divinity and escape back to the real world.

A bit extreme version:

Dark divinity wants to break/go back into the world of the living. He finds this child that was borne with the soul that he can overtake through some ancient ritual. So one night when boy was playing with his sister divinity pulls boy's soul out of his body living him to be an empty flesh-shell. Sister is in shock and also no one believes in her story so she decides to get him back herself.
Brothers soul manages to create mind connection with her and instructs her to go to the old ruins close to the edge of the town. There she needs to kill herself in order to follow her brother, so she drinks the vial of poison near the ancient well that is portal to the lands of the pagan god.
They both has to conquer the divinity in order to regain their souls and resurface back to the real world.