Ideas

Main Character
A pair of siblings who are chosen for a ritual that only needs/allows siblings to be used as both priest/priestesses and sacrifices depending on who survives.
They are close though and plan to work together in order to make it where they need to make it to in order to survive. 

Other Characters/creatures
A god who created this ritual in the first place in order to fulfil an oath to something else, something who only takes payment in the form of life - specially bones. 

Fantasy land/world
You can bury your secrets, but they come back around as a plant, one that will entice someone to come closer to them in order to whisper your secret. In this only the desperate, those that can't keep the secret to themselves, does this and in very secluded or hard to reach places in order to limit the spread of the secret. 
Bones or other forms of life can be used as a barter system with a few creatures and the other god, in which case bodies are not buried but rather interred in something like a crypt or mausoleum. You can bury a bone or lifeforce imbuded item into the ground as payment to the god in order to ask a question of the dead, if you want a speific person to answer your question you need to know their name and need to invoke that name as you bury the thing as payment. 

Contrasting Obstacle
The main point/part of the ritual the siblings are involved in is that the siblings compete against each other, so they start in random, separate locations. One or both gods interfere in the ritual even as they try to work together in order to enforce the oath and sacredness of the ritual.  

What Could Happen
The siblings are forced apart for a ritual that could be their death the only true way for both of them to survive is to find out why the ritual was created in the first place and use that information to break it. The problem though is that it was created millennia ago so all sources of information is in ruins and with the ded leading to the siblings trying to decide what they're willing to give up or sacrifice in life force/bone payment to the god in order to speak to the dead. One of the siblings come upon a plant that whispers a secret in the distorted voice of their sibling, it's in the ritual grounds that peopletry to avoid when it's not in use, the most daring and hidden place one could plant a secret.