I found this out accidently today when I was looking at a family maxis uploaded. Now I'm talking about on the in game gallery not the online version however I"ll link you to the online version to see the household I m talking about as I'm not in game now. Anyway you go to the right side and mouse over the sim and it tells what relationship they have to the other members. Today I saw brother and sister of. If sims aren't related it doesn't say anything.
https://www.ea.com/games/the-sims/the-sims-4/pc/gallery/DE808F69A25F11E8819236427980D667?category=all&searchtype=ea_origin_id&sortby=newest&time=weekly&searchquery=maxiscreator_01&max=50&maxis=false
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Really? I haven't noticed it till now. Maybe it's because I use the online gallery more.