Not related to any particular game, just lore-wise: is the Dark Brotherhood able to refuse a contract to kill someone? Like, what if killing a particular person is not in the Dark Brotherhood's best interest, or is a member of the thieves guild or some other friend or ally?

Assuming the black sacrament was performed

If so, does the reason supplied need to be sufficient for the night mother or Sithis?

If I had to guess, I would say yes, but only if another life was taken for Sithis to replace the victim who otherwise would have been killed.

P.S. If this thread is unnecessary, could someone please direct me to a dark brotherhood discussion thread? thank you.

One thread is enough: http://www.gamesas.com/topic/1523696-can-the-dark-brotherhood-refuse-a-contract/