Um, it's talking about the age Ella was at the time of Stephanie's death and Stephanie's own age at death, therefore one can hazard a guess as to how old Stephanie would have been at the time of the wedding.
Ella wasn't born at the time of the wedding so she certainly wouldn't be flying anywhere, least of all with a mother and stepfather.
By working back from Stephanie's age at her death and when it was by taking Ella's age then, one can arrive at a guess as to the age of Stephanie at the time of the wedding. Eg I seem to remember that Stephanie was about 15 when she died and, if Malice was pregnant with Elsie, Ella would have been three or four at the time. If Ella had been born two years into the marriage, ie five or six years previously, then Stephanie would have been around 9 or 10 so perfectly able to attend. The ages are a rough guess. It's just maths.
