Lynne Wright, I like you can only go on known facts. 1891 census clearly states that Isabella is the sister of David-so must share at least 1 common parent or both, unless she was adopted. Georgina born 1901 is David's niece.
Working on that and having found David's birth record -parents Bernard & Ann McCullum
Name David Cumming
Gender Male
Birth Date 15 Sep 1862
Birth Place Edinburgh Parish,Edinburgh,Midlothian,Scotland
Father
Bernard Cumming
Mother
Ann Mc Callum
ADDED: David Cumming (NO S) birth certificate shows him as "Illegitimate" son of Bernard & Ann McCullam
So if David IS the son of Bernard & Ann then other children ie Bernard jnr, Susan & Mary Ann are his siblings and in theory siblings/half siblings to Isabella/Ann Isabella.
I cannot give you an answer as to why there is no birth registration, a seemingly 26 year gap until she appears in 1891 and then goes off the radar. She allegedly dies between the 2 marriages of Georgina her daughter but I cannot find a suitable death ref for her.
SS