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!