Hi Rosie
Yes I would have though that the marriage record would be right with her sister Ann Elder as a witness, The other though I had was that the 1854 birth date could be a fudge and maybe she was born a bit earlier before the marriage of David and Annie in 1853. So far I have not found a birth record for Helen or any info on Elizabeth Drummond in the Opr's or official records.
Regards Smid