Puzzle n°46 (*) - The russian family

Let's meet for once a different breed than the somali : the Russian, often also called "Russian blue" because many cat federations only recognize this breed in the blue colour which was the original one of the breed. Russian cats are elegant and have fascinating emerald green eyes. Once upon a time, three Russian cats of the same family had a brother in common. Unfortunately, this brother died. From then on, each of the three Russian cats had no brother any more. How can this be ? (no need to search for half-brothers or other complicated matters) ... I'm giving up...

(picture © J.P. & A. Debus)