Hi LittleLondoner,
On SP you can only view certs upto a certain date.
So Barbara and Jane's birth certs and any possible marriage certs will have to be ordered. Although as you already know their birth dates and parents names you won't get much more information from them anyway.
Elaine gave you the marriage details of their parents James Begg and Margaret Montgomery. From this I've found the marriage details of James Begg's parents. David Begg and Jane Cowan.
David and Jane married 23 Jan 1880 in Glasgow. Davids parents were David Begg (farmer) and Janet Gunn.
Jane Cowan's parents were Alexander Cowan (dairyman) and Janet Keith.
Witnesses to the marriage were William Begg and Margaret Cowan.
David died 20 Nov 1908 aged 52, Glasgow.
Jane died 11 Feb 1895 aged 38, Glasgow.
I don't know how much use this will be to you. I'm not sure how far back you wish to go or if you are just looking for more recent relations??
Sueky