My Sister died a few years ago. As she left no will and had no husband/partner and no children her estate went to probate. As I was her only living sibling I was made executor of her estate. I received my share and the rest was divided between her late brothers and sisters siblings. I have since found another of her savings books, which is in joint names with one of her nieces. If I hand this book in, will her niece get all the proceeds of what's in the account ?
Yes. Joint owners inherit property irrespective of the terms of a will/intestacy. Unless it was put into her niece's name for say, convenience, to help her pay bills etc (unlikely if it is a savings account) which is difficult to prove the account rightly belongs to the niece.