Use the search tool...there are a zillion threads on these topics, as people have had the same questions before. Be sure to use "advanced" panel so you can limit search to the Prep School subforum.
While you're at it, may as well bump some of those threads by putting a new post in...this way more people are likely to see it on the first page.
Yes, your sibling can come. We dragged our younger daughter along to nearly all the campus tours/interviews a few years ago.