A FRIEND IN DEED might be the only one where the first killing might or might not be an accident, and you never find out.
This is going a little far afield, but there's also at least one where the MOTIVE might or might not be based on a big mistake, and that's TRY AND CATCH ME (I know a lot of you are sure that Edmund murdered Phyllis, but some of us keep wondering).
Is the same thing true of THE CONSPIRATORS? I don't know it well enough to know whether it ever tells you for certain that Devlin was being cheated by Pauley.
I've always wondered that about The Conspirators myself. From what I can figure, Devlin has never met Mr. Pauly, since he didn't recognize him at the bookstore, and therefore didn't know how Pauly actually runs his gun deals. Maybe he sets them up, takes the money, stores the guns someplace and has his hired help make the delivery while he has left town to avoid being implicated if the deal gets busted by the police. Devlin is making assumptions that since Pauly bought a new suit and a plane ticket that he was skipping out with the money instead of conducting his business like he always does.
I'm sure someone can correct me on these right away if I'm wrong.
In the original series at least, there are three murders of siblings, ANY OLD PORT IN A STORM, OLD-FASHIONED MURDER and LADY IN WAITING. I kind of doubt that the show was trying to "say" anything by this, but there were none committed against sisters, only brothers, but two where the MURDERER was a sister.
Also, there were three killings of uncles by nephews, DOUBLE SHOCK, SHORT FUSE and SUITABLE FOR FRAMING, and one killing of a nephew by an uncle, THE GREENHOUSE JUNGLE.
Like a lot of you, I've never managed to really get "into" the later version. But JACKPOT is one of the exceptions for me, so I'm sorry I didn't think of it.
It took me a while to realize which episode of the FIRST version you meant, but now I see what you mean.
I don't know about the later version, but when it comes to the first series, there seems to be only one murder of a mother-in-law. Which is interesting, considering all the traditional jokes about that particular kind of murder!