Romantic Relationships among close family members

I have a family that has since moved into different lots/households. I noticed my sims sister was outside her house so I went to talk to her and saw the romance bar. I was confused and thought maybe somehow they became unrelated or that it was a duplicate of her sister, but then when I clicked into their profiles it shows they are siblings and have a slight romantic relationship. I have no mods and I own all packs, except for a couple kits.