Why would you do this and say this?

This guy I like is a family friend. He messaged me in the beginning of quarantine. He started sexting me and texting me. Next thing you know it's going on for 10 months. We talk like multiple times a week. We have sent each other nudes, videos, and told each other intimate details. His older sister and my older sister are friends. He recently asked me if she's seen our texts. Before I could answer he goes, "probably". My response was, "no, I'm private like that, but she knows we talk." His response was, "oh ok awesome". He then continued talking to me normally. Why would he message me for so many months and then ask this randomly? Thoughts?
Why would you do this and say this?
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