What if someone prayed to the nightmother to kill a member of the dark brotherhood?

I imagine they'd be subject to the punishment of the Dark Brotherhood, where you fight that wraith in a dream (what is that called?). That's just my opinion.

The Wrath of Sithis. And it's not in a dream.
I'd imagine it would be ignored, depending on how high up the brother/sister was in rank.

thats what i was thinking, but it just seems kinda contradictory

Perhaps if it is a Speaker or the Listener himself then the person will be killed, but Silencers or below will be scrutinized heavily for wishing that?

I imagine it would be ignored. The Dark Brotherhood would be contractually obligated to kill the person, but are prohibited from doing so. So I think the Night Mother would never act upon the request, under normal circumstances.

Why not just outsource the contract?
...it circumvents the prohibited aspect of the request

I bet they just ignore it.

I believe by the transitive property, that would be breaking Tenet 5: Never kill a dark brother or dark sister. To do so is to invoke the wrath of Sithis.

Its safe to say DB members to not pray to the night mother about the murder of other members.

I believe he was approaching it as... let say brother and sister in some random farmtown. Sister wants brother dead to take over land. Sister prays to The Dark Brotherhood to kill her brother. Unknowingly to her, her brother is a member of the Dark brotherhood.

Therefore, problem arises.

Ha!

Well I'd say either brother finds out and carves up sister, or sister gets recruited to join DB, or maybe the prayer just goes unanswered.