There is the following sentence in Jeffery Archer’s detective story False Impression:
Arabella was so wise and sensible. If only her beloved twin had been born a few minutes earlier rather than a few minutes later, then she (Arabella) would have inherited the estate (of Earl of Wentworth titled in 1815) and undoubtedly handled the problem with considerably more panache.
From the above sentence, I speculate that the twin who was born later than the other is acknowledged as the elder sibling and the first heir of the parental estate in order in U.K. If my guess is right, what is the reason, base or provenance for giving priority to the “late comer to the world,” not the “first comer’ in the family order?
In Japan, we had the same custom of acknowledging “the born-later” as the elder sibling before the Meiji era (up to late 19 century), but today’s Family Register Law provides the “earlier-born” to be the elder of the twins.
Update with more context
I have added the previous paragraph which makes this sentence Victoria's point of view about Arabella.
Victoria looked steadfastly ahead as she climbed the wide marble staircase to her bedroom on the first floor. She felt unable to look into the eyes of her ancestors, brought to life by Romney, Lawrence, Reynolds, Lely and Kneller, conscious of having let them all down. Victoria accepted that before she retired to bed she must finally write to her sister and let her know the decision she had come to.
Arabella was so wise and sensible. If only her beloved twin had been born a few minutes earlier rather than a few minutes later, then she would have inherited the estate, and undoubtedly handled the problem with considerably more panache. And worse, when Arabella learned the news, she would neither complain nor remonstrate, just continue to display the family's stiff upper lip.