(Ok this is gonna be weird but a cursory look doesn't turn up any credible sources.)
I'm asking because of my previous question: Can co-parents-in-law marry? For example, a husband's father and a wife's mother (but applied to Catholicism)
I assumed that step-siblings could not marry in Catholicism (or otherwise).
Based on this assumption, I questioned the difference between step-siblings' becoming a married couple and a married couple's becoming step-siblings.
Actually, apparently, step-siblings can even marry in other denominations and religions and even in civil law apparently. (Note: In the original question that my previous was based on, I wasn't able to see anything in the answers that even say 'Oh actually step-siblings may marry.')
Question: So, can step-siblings marry in Catholicism?
If yes: then oh Hell insane but whatever.
If no: then ok then back to the other question. (I didn't really analyse the answer and comments so thoroughly because I wanna double check this assumption 1st.)