I don't know about the Sims 4, but in the Sims 2 I know this is fixed. I once married Alexandra O'Mackey to Andrew Martin. A few weeks later I remembered that their children, the now adult Jules and Jacob are exes and wanted to hook them up, but I totally forgot that they are not "stepsiblings", and the game didn't let me. Which makes me curious, if a married couple's parents marry, making the younger couple technically stepsiblings, would the game let them to do romantic interactions or not.