(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: According to Catholicism, can "co-parents-in-law" marry?
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.)
Edit to clarify based on Andrew Leach's comment:
I mean suppose Alice and Bob are widows (or single adoptive parents or unmarried biological parents or whatever) and their respective children are Charlie and Dalia. And then Alice and Bob get married. The question then here is about Charlie and Dalia.
I'm not talking about cases like say Adam and Barbara have a biological child Carl and then later Adam and Barbara adopt Danielle, where I'd ask about Carl and Danielle. Or simply Adam and Barbara adopt 2 children. So not that. I mean to ask about the situation above with Charlie and Dalia.