The Irish registration indexes do show a Rachel Blood born there in 1883 (
http://search.labs.familysearch.org/recordsearch/ ), who could have been the "half" sister your mother remembered. The split between James and Esther probably happened sometime 1883-1891? Maybe earlier
Probably the "George" on the marriage certificate was a mistake as Esther Jr may not have known/remembered her father very well, and she did have a brother named George.
Probably the best thing to do would be to try and look at army records. Best to actually go to the NA as there are issues with exactly what his name was - but you do have his regiment (I think 1/13th = 13th Light Infantry = Somerset Light Infantry, but army records are not my speciality so best to doublecheck that). If you couldn't find his discharge papers you might still be able to track him in muster rolls etc.
The other set of records, probably at the LMA, would be the records for the school that Esther Jr and Rachel were at in 1891. It would be useful to know when they entered the school as that will give you some more information about the family's whereabouts between 1881 (1883?) and 1891. If you're very lucky there might be some information about family circumstances/location of parents.
