Erm, if that bloodline is something that can be passed on both patrilineally and matrilineally, then there is no way those two are the last descendants, there would be people that crystal reacts to all over the place. If it's patrilineal only, then those two will not be able to continue the bloodline anyway unless they somehow gender transition in the purest way possible, as in gain the Royal line y chromosome from somewhere.
So how the fuck does that crystal work?