When did owning a smartphone mean that your friends and family were owed access to you at every minute of every day?

I helped someone out with something last night, answered a follow up email, and didn't see their 2nd follow up email until this morning. I emailed back, the person emailed me the answer, and I haven't sent my real feedback on the follow up question yet because I have a lot going on at work. The person just forwarded me their email from this morning - as though they are chiding me for not answering it in the intervening 4 hours. This is the second or third time this person has done the same thing. (For clarity's sake, this is a family member, not a client or my boss or anyone like that. And it's a non-emergent issue.)

I find it infuriating. The fact that I own a smartphone doesn't mean that other people are entitled to my time. There is another family member on the same side who does the same thing, and it drives me crazy. I am a GROWN ASS WOMAN and I don't owe anyone an explanation of what I'm doing with my time (except my clients on my bills and maaaaybe my husband, so he doesn't think I'm dead in a ditch somewhere).

Who is with me? And what is a tactful way of telling these people that they are being pushy and it doesn't sit well with me?