Isabella Magdalena Susanna Gray (Doubell), c5 - Birth Year?

At present - 1864 (taken,I think, from her death certificate that gives her age as 58 on 7 Sep 1922)
but her sister Aletta Maria Leon Harth, c4 has a birthdate of 2 March 1864
Winterhoek, South Africa. No documentation; although the date seems pretty specific.
Were they twins?
We need more sources to figure it out.