I checked, this is my large rotational save that has a lot of intermarrying, my cousin married your sister kind of thing. Here is what I have discovered: The more confusing the intermarried ones ar...
As far as I can tell with the profiles, if they are close enough not to need the word "distant" in their connection, including relative-in-law, they show up in the family category of the relationship panel, and the relationship shows up in the Sim's profile:
And
If they are "distant" relatives or "distant in-laws", then the relationship is not in the profile page (as far as I've seen). Definitely keeping all those second-cousin custom labels. I can see why they did this, because in some cultures it is not only acceptable but encouraged. Also, growing up in the South, second cousins were sometimes called "kissing cousins," which I assume meant it was okay to flirt and kiss as long as it didn't go further. I didn't grow up knowing many of my second cousins beyond the occasional photo, so I wouldn't necessarily know them if I passed them on the street. Also, there was that one New Girl episode where she just happened to discover that her boyfriend was her second cousin; she was very happy they had been taking things slow.