Sending likely cannot contact the soul
You send a short message of twenty-five words or less to a creature with which you are familiar.
Creature: It is unlikely to work, because sending needs a creature to target. The former creature they are trying to contact consisted of a body and a soul,
- the body now that they are dead counts as an object
- the soul of a creature is not the same as the former creature. Whether a soul or spirit counts as a creature at all is not entirely clear, but I think it does not, unless it got turned into a ghost, specter, or similar entity.
So the spell would not work. What the caster knew as a creature to contact now is part object and part up to the DM, but probably not a creature.
Familiar: the body of a creature can be polymorphed, petrified, and exchanged in many other ways by magic such as the Clone, Magic Jar and Reincarnation spells. For example, if your character's soul were to inhabit a new body from Clone, and you were contacted by Sending, no matter if the old body lies rotting in a dungeon somewhere, you would expect the sending to reach you.
If the DM rules that the soul is really what makes the essence of a creature, and souls count as creatures, sending also demands you are familiar with the creature, and this is a hurdle the DM will need to allow you to clear, too. Are you still familiar, when the target has changed in such a substantial way?
Lastly, whether alive or as souls, it certainly would be within Asmodeus's power to block sending (it is not a divination spell, so normal anti-divination measures do not work, but Asmodeus is a deity).
So, in essence, you can decide if you want them to be able to reach their relatives, or not, but the rules would point to no.