I am a bit confused. If Elizabeth Cully (13 yrs old in the 1911) has a birth certificate that gives her mother’s name as Jane Morrison and her father as Andrew Campbell, then why is her mother named as Elizabeth in the census? And why is her surname in 1911 Cully and not Campbell? Something’s not right. Either she’s got the wrong birth certificate, or surely we have the wrong family in the censuses?
Do you know for sure that your sister did have a younger sister Elizabeth, and an older brother Thomas? (That will help decide whether the 1911 family is the right one).
Looking at the LDS pilot site, I can see births for all 3 of the children named in that 1911 census.
Thomas James Cully July – Set 1889 Belfast Volume 1 page 340 Caroline Cully Apr – June 1891 Belfast Volume 1 page 387 Elizabeth Patterson Cully July – Sept 1898 Volume 1, page 238.
In each case there was only 1 birth in Belfast at that time that fitted the census info, so am reasonably confident about them.