Jacob lives in Carlisle, Iowa, with his mom, Mariana Andris, and stepdad, Mike Ellison. He doesn’t have any siblings (though that doesn’t completely rule out that he might someday). His mom remarried about a year before the events of Bowman of Wellwood.
I’m betting both Mike and Mariana would be completely shocked to learn that Jacob’s best friend is a four inch tall sprite. ^^; Mike is very pragmatic, so he’d probably be quietly wondering how this is possible at all while Mariana, once she got over the initial shock, would probably love Bowman. She’s a sweet woman who happens to love tending to plants, so she’d be happy to meet a plant person.
Both of them would respect Bowman’s secret, but Jacob hasn’t told them about his small friends out in the woods just in case.