At most, we see hints that Momo actually likes Ao more than just as a sibling. However, while Ao does love Momo (only admits it in his sleep, etc), it's more of a familial love. In later chapters, we see that Momo gets visibly jealous (when random girls get close to Ao) or just conflicted (when Satsuki-chan makes progress with Ao). Momo even dreams of Ao relying on her and being all clingy with her. Ao thinks she's just being immature or irrational most of the time.
On an unrelated note: Orihara-sensei finally gave us that Futago Complex/Policewoman and Assassin crossover I joked about last month. I couldn't be happier.