Honestly, I haven't got the slightest clue. I've googled things like "British nicknames", "British nicknames for siblings", "British nicknames for younger brothers". I usually just keep getting British nicknames for people from a specific place in England or nicknames for British army units.
I know that, for instance, my little brother calls me Tata, which is obviously not my name. But, we're Cuban, and in our culture, generally, the older sister is called Tata. Just a nickname like saying "Mum" or "Dad". I was wondering if perhaps the same thing existed for the Brits.