@yannickq, the way I read it is that the Feschers and their ancestors were for the Empress and her ancestors what Tia was for Lawrence in the previous timeline and for Cedric in the current one: their spymasters and fixers---who guard the imperial family from the shadows. Their ownership of St Olga's Heart symbolises this role.
In the present generation, the Feschers "lost" badly to (probably) Gregor---a crown prince by adoption---who succeeded in having the Empress's children assassinated, and they hung themselves in shame---or perhaps were hanged. This left the Empress without eyes and ears, trapped in her palace, so Tia, St Olga's Heart, and Cedric only really came to her attention after she read about it in a newspaper---when Tia's beating and Cedric's rescuing of her, along with the human trafficking bust, made headlines. Thus, Tia got her audience.
When Tia gives St Olga's Heart back to the Empress, the Empress first assumes that it is merely a naive, ingenuous gesture, gifting the empress her deceased friend's jewellery. Thus, the diamond in question has no meaning in Tia's hands---so the Empress thinks---and there is nothing to it. In fact, in Empress's hands, it represents Empress's own failure to protect her children and her loyal servants.
Then, Tia shows that she understands the significance of the Feschers and the meaning behind the St Olga's Heart and offers to take their place. Cedric understands some of this---though he was probably too young and isolated from capital affairs to know about much of the background. Tia then sends Cedric away, so he isn't there when she and the Empress plot treason.
Does this sound right?