Hi Erica, Jonathan and Danielle! Thanks for your replies - two from the USA and one from England! I never expected this! And, what's more astonishing is that all three of you are related to me - and we would be related to each other! So, hi 'cuzzins'! :-) Erica is my 19th cousin 4 times removed; Jonathan is my 19th cousin 5 times removed and Danielle is my 6th cousin once removed. Thanks for the links - I will try to figure out how it 'works'! I reckon I can understand why Danielle and I are 6th cousins once removed. Our pivotal connections are Martha Du Plessis (my 4th g-grandmother or 5th grandmother), and Pieter du Plessis (Danielle's 6th grandfather or 5th g-grandfather). Martha and Pieter were sister and brother. Or do we have to go back to Martha and Pieter's father and work from there? He would be our most recent common ancestor.
To refer to Jonathan's comment: The distance between myself and the common ancestor is shorter than the distance between Danielle and our common ancestor. (By one? Is that what 'once removed' means, or is the fact that we are dealing with a brother and sister 'the once removed', i.e. the distance between the brother and sister?). Maybe I will understand all of this better once I have read the articles!