Pls dont tell me the little brother will have feelings for her

Not incest involve. He truly loves his sister very much and nothing more

i think they meant the ml little brother and not the fl big brother

i think it's more of an esper-guide attraction for ciels brother, considering the fact that he's never met a guide before (when she accidentally let's hit the aura) & he's not used to being treated nicely by others in general