If you scroll down through in the JM Barrie section in this forum, you will see many posts by descendants of his siblings or other relatives, who might be able to help you in your research. One of them would be Tanya Vaughan, a great-great-granddaughter of one of JMB's brothers, who inherited a lot of archives from her grandmother. The other person who could also be helpful is Sandra Affleck of the Kirriemuir Heritage Society who should be able to point you in the right direction, as she has done a lot of research on the Barrie and Ogilvy families, as well as know many of their descendants still living in or near Kirriemuir. (Both Tanya's and Sandra's emails are listed in the members' list.)
Good luck with your research and please keep us informed of your results. Are you writing a book about Barrie and his family?