Can anyone tell me about this manhwa please

The bottom is a guy who has a fwb of sorts and stuff

And suddenly one day he gets a step sibling (new top) who has black hair and blue eyes and a dark and mysterious aura

And no one really realises the person and stuff for a while

But then when the mc gets confused
The next day
Everyone starts seeing the new guy all of a sudden

And the old top who is fwb with bottom feels weird and doesn't understand him

That's all i remember please tell me the name if you remember