There's more to it than that. To sell books in their store you also have to get an ISBN, which costs money. If you wanted to sell books to just a few people, like your family, you would still have to spend the $100 odd for the ISBN, which would be a bit ridiculous.
[Edit: looks like I was wrong there, you don't need an ISBN for a free book if you have a free book agreement with Apple]
You are basically wanting Apple to provide free software (which they have), and a free platform for you to distribute what you make in that free software (which they have, at least it's free if the books are), and then you're not satisfied with that, you want to be able to customise the platform so you can direct your books to just a few people. I don't think it's likely to happen - Apple are in business to make money, and I can't see a lot to be made like that, even if you did charge for the book.
I am making an iBook version of a class lab book right now, that I want to be able to distribute to just 30-odd students (actually not even that many, they don't all have iPads) and yes it would be great to be able to do it in the iBook store, but I can see that it's not going to happen, and I don't expect it to. It's enough to have the ability to make the book with video etc, I can figure out how to distribute it.
Now if only the thing would stop behaving erratically, I'd be happy.