Isabelle is the youngest Holmes sibling, Sherlock and Mycroft's little sister. But they never knew of her and she never knew of them. She was quickly given up for adoption when she was born after the horrific events the parents had with their other daughter (also unknown to Sherlock at this point). What happens when her adopted mother tells her about her crime solving brother and government leader brother. She goes to visit Sherlock and experiences new adventures on 221b Baker Street.