Hi,
The only one still alive is Margaret (Peggy) who, unfortunately isn't well enough to talk to about all this.
I have the birth certificate, or have viewed the birth certificate for every other sibling except Jean. I've searched for J Forrest, since I now know her year of birth but still not finding anything.
I don't know for sure, but i'm guessing she was born in Kirkintilloch as all others were born there too, but have searched for all of Scotland just in case.
Parents were James and Margaret, married in 1907.