Ehhh, I want more crazy brother-sister interactions, I don't want a third character to "play" with them in a way that distracts from that. Well, hopefully I'm jumping to conclusions.
That's just a joking way of saying it, but if you want manga examples then she could time loop, become a ghost, become a zombie, isekai, same-world reincarnate, become a participant in Death Parade, etc.