Aha! Another bookaholic!
I know it's hard but the first thing is to sort out your book collection and trim it down. Give away to friends and family or charity shops. If you wish to make a little money on the unwanted books, hold a yardsale or sell them to a secondhand bookstore. The latter may only accept certain books - the popular ones they can resell.
What's left can go on shelves - either floor bookcases (if you have the space) or actual wallshelves (for smaller lighter ones). Where these shelves can go depends on available rooms - bedrooms, rec rooms, family rooms. Apart from your bedroom, the other choices will naturally depend on your sister and her husband. If you have a lot of books at this stage - go for tall bookcases.
Other novel ideas for books are shelves across the top of a doorway or all along the walls of a room - at sort-of picture rail height. Cookbooks should be in the kitchen within easy reach. If you have beautiful coffetable books then the living room for all to admire. Don't forget the bathroom! - A basket of "bathroom readers"?
If you are truly out of space but still have a few more books that cannot be displaced, put them in underbed storage containers for now.
Hope this helps and that you will accept my ideas. Please do ask me if I can be of further assistance.
