[Kalimac]

Just looked at the genealogical tables in ROTK, and as far as I can make out (and I might be missing some glaringly obvious point) Drogo and Primula were not technically related directly to each other at all. Both were related to Bilbo, but that's a different matter - for example, if I'm descended from family lines A(mother's family) and B (Father's family) and my maternal cousin is descended from lines A and C(his father's family) HIS cousin (descended from lines C and D) has no blood relation to me even though we have an indirect connection.

On an unrelated note, I'd like to point out that a one-off first-cousin marriage is not a genetic problem in 99% of cases (the 1% being where the common family involved has Tay-Sachs or hemophilia or something like that in their bloodline). It was repeated first-cousin marriages of children who in turn are products of first-cousin marriages that usually did the mischief, and those were usually made only in royal families for the sake of alliances - the old church, in fact, officially condemned marriage between kin closer that *seventh* cousins (obviously a lot of dispensations were handed out). So even if Primula and Drogo were second cousins by blood, the risk to their offspring would have been very minor.