Hi Martin,
here is some explanation from me Hope it will make sense.

It has already been said that Svetlana considered eleven to be her special number. In fact, her short forty four years can be divided into equal quarters like a chessboard. When Svetlana was eleven her older sister went away to a big city and later got married; about eleven years later her mother died; about eleven years after that Svetlana married, became a mother, and her father died; and when she was forty four she herself left this world.

Как уже упоминалось ранее (как уже говорилось), Светлана считала одиннадцать своим особым числом. И действительно, её короткие сорок четыре года жизни можно, подобно шахматной доске, поделить на четыре равные части. Когда Светлане было одиннадцать, её старшая сестра переехала в большой город, и позже вышла замуж; приблизительно одиннадцать лет спустя, умерла мама Светланы, а ещё через одиннадцать лет Светлана вышла замуж, стала матерью, и в это время умер её отец; а когда ей исполнилось сорок четыре, она сама покинула этот мир.

It has already been said – как уже упоминалось ранее. It is something like: “As it was mentioned before…”. Or more word-for-word translation would be “Как уже говорилось”.

“Особое число” vs. “особенное число” truly said I don’t see any differences in these two words, but somewhere in the depth of my mind I feel that “особый” will be more correct 

“In fact” is correctly translated as “На самом деле”. But I have used an expression “И действительно” to make more definite logical cohesion with previous sentence. In English it should be something like this: “Svetlana considered eleven to be her special number, and really, her short….”

Truly said, I don’t see any problems with equal quarters that chessboard could be divided into. So it can be freely translated as “подобно шахматной доске”.

Correct translation of “equal quarters” is “равные четверти”, but we very seldom use such expression. It will be better to say “четыре равные части” (four equal parts).

Sometimes it is more useful to translate word-by-word, as the next sentence:

when Svetlana was eleven her older sister went away to a big city and later got married
когда Светлане было одиннадцать её старшая сестра переехала в большой город и позже вышла замуж.



about eleven years later her mother died - I translated it as “приблизительно одиннадцать лет спустя, умерла мама Светланы» It also could be translated as (умерла её мама or её мама умерла), but before that we were talking about Swetlana’s sister and if we say её мама умерла it can be understood as if Swetlana’s sister’s mother died. Logically it is also true, but all our text is about Swetlana (not her sister), so it would be better to say that Swetlana’s mother died.

about eleven years after that – ещё через одиннадцать лет.

and when she was forty four – а когда ей было сорок четыре. This translation seems more correct, but I feel myself somehow uncomfortable when I listen such sentence. So I used the word “when she became fourty four”

So the translation and explanation as for me is something like this. I’m not a philologist of Russian, so I could be mistaken somewhere. Nevertheless, as I’m native speaker, I would translate this text in this way.