The bard in my group lost his mother and 2 sisters under mysterious circumstances. Unbeknownst to him (or them) his father traded the wife and children to Asmodeus in exchange for success in business.
The bard just turned 5th level and has announced that next session he is using Sending to contact his mother and/or sisters.
I haven’t completely settled on whether or not they still live a tortured existence as playthings for Asmodeus’s favored minions or if they are dead and their souls are continually tormented. (Very dark part of the campaign, I know.)
If they have been slain will the bard’s sending be able to contact them or does Sending only work on the living?
Sending would not work for contacting the dead
The first line of the spell:
You must choose a creature to hear the message. Since death causes a creature to, in game vernacular, change from a "creature" to an "object", with regards to targeting, the spell will fail.
However, you did mention in the question that they might not be dead. If that is the case, it should work barring other problems: