Thanks for the replies. So I have a birth record for Margaret, which records her birth and baptism in Huntly. I also have her marrying a John McDonald and then a death certificate for her in 1938. Interestingly, both her marriage and death certificates cite Christina as being her mother, but the marriage certificate has her father's name as Edward Forbes (presumably a mistake) and there is no father's name on her death certificate. Her death is registered by her daughter, Margaret Forbes.
I previously tried to find the birth details but drew a blank, hence my post from a few years ago. Because of all of the illegitimate children, the marriage dates and the various surnames, it's quite complicated to piece together! My great grandmother is Margaret's daughter, Jessie (also known as Janet) Forbes.