Sister Crimson and Brother Segarius have a Ravenous Earth-like ability, but these do not seem to inflict damage. Have not yet tested if this is the case for other RE-like abilities on monsters.
In case of sister crimson, this volcano is a summon. From what I can see on grimtools, it’s supposed to do aoe damage, probably around itself? Either way, this damage is just laughably low (265 flat phys and 487 flat fire at lvl 110). Changing whole skill to proper volcano skill definitely wouldn’t hurt.
For brother segairus… I could swear he had some sunder, probably on volcano too, but I can’t see it on grimtools now? Was it removed at some point? He could pack a punch just like sister crimson in the past, but doesn’t seem to anymore.
Tested it some more, it does deal damage if you stand almost on top of it, but it is almost not noticable.