OTMA

OTMA was an acronym sometimes used by the four daughters of Emperor Nicholas II of Russia and his consort, Alexandra Feodorovna, as a group nickname for themselves, built from the first letter of each girl's name in the order of their births:

In childhood the grand duchesses came up with ОТМА as a sign of sibling closeness and affection for one another, writing it in their diaries. The girls were great granddaughters of Queen Victoria and, although "thoroughly Russian," grew up speaking both Russian and English fluently among themselves. Whilst the family was in captivity after the Russian Revolution of 1917 they were allowed to send few letters so the sisters often signed this nickname on cards they had written together for loved ones and friends.

...
Wikipedia