Very many thanks Mary, for looking up the entries. It is very strange that a twin wouldn't be entered (or baptised) at the same time as her sister, even if sickly. It seems more likely that the curate simply forgot to record a second child.
I have seen a bapt for twins where one twin was (assume later) entered in the margin next to her sister's entry, and in this case it might not have been picked up because she didn't marry in the IOM.

Sue