The OlderSister's diary is found in the Elona+ mod of Elona. It can be randomly attained in treasure or lying around in a dungeon.
Using the book will display the text, "How...! You suddenly get an Older Sister". After this, an Older Sister will join you. If your party is full when you read it, the book will still be consumed.
The item sprite ID for the book is 429 (Row 13, Column 0) using the item sprite zero-based position system. It is tinted randomly like a normal spellbook.