Isobel was born in 1899. She is the daughter of Thomas Hall and Mabel Harrison. The name of her aunt, Isobel Hall appears as a witness in the baptism register.
Sources
FamilySearch: "South Africa, Church of the Province of South Africa, Parish Registers, 1801-2004," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:QVS4-N6YF : 16 August 2019), Thomas Harold Hall in entry for Isobel Mary, 08 Nov 1899; citing Baptism, Cape of Good Hope, South Africa, p. 13, William Cullen Library, Wits University, Johannesburg.