LM,
I have drawn a blank to on the Olive Family - unless Olive has been transcribed wrongly - I have seen Oliff, Ollive, Cleve, Clive etc. etc.
I have, however, laid claim to two brothers for James John Olive, based on parents names (John and Mary), geographical proximity to Marylebone and within time frame - William and Thomas - baptised at St Pancras in 1798 and 1780 - and hope that at some point maybe there will be a DNA match through their descendants lines.
Mary will have to remain a mystery sadly!
EM