No, that's exactly what it is not. 'Sibling group' implies that there is no direct line of descent, but that at some point the tree of life forked, that Comb Jellies are in the one fork and that all the rest of the animals (including us) are in the other.
> The LUCA is not the first life on Earth; it may have lived among a diversity of other organisms whose descendants all died out. Rather LUCA is the most recent form from which all surviving life on Earth is descended.
If this stuff interests you I would like to recommend Dawkins' 'The Ancestor's Tale', it is pretty accessible and walks you back through time step-by-step.
