I've just run upstairs for you, and grabbed my copy of Andrew Birkin's excellent book J M Barrie and the Lost Boys.
JMB's elder sister was Jane Ann. His younger sister is referred to as Maggie but, of course, that might just be a contraction of Margaret.
If you're looking for a family tree, his father's name was David. His mother was Margaret Ogilvy. (She retained her maiden name, in accordance with an old Scots custom). Their oldest son was Alexander. Their second son was David (who was killed in a skating accident, on the eve of his 14th birthday, in January 1867)
Chris