I would have spoken up, on the spot and asked what's the deal with that. Even if in a playful way. Maybe his mind wasn't on the answer in that moment and your question led to him thinking on it more so by the time your sister asked he was more focused. At the same time, it's important to point out the discrepancy in conversation. I mean wth, as a parent it's imperative to try and give equal or close to equal attention and conversation.