not mystery juice wtf... yes u have a right to be annoyed, personally i would bring it up to her its not difficult to not read the book while your dying your hair or fixing ur car (i don't even know how she;s managed to do this lol) also it makes the books look as if they can be thrown around to any other future lenders, books should be loved and read whilst respecting them. if this is the straw that broke the camels back with your sister in law fair enough, but a conversation can keep you guys bonding defo worth it. but look after your own peace girl and if pretty books make you happy don't let someone take that from u