SPOILER ALERT
Her father doesn't remarried but he is a bastard who play around with other women. Ekaterina and her brother share the same biological parents so far in the novel, her brother was taken by their grandma when he was little cuz he's a succesor. The previous duke is not their father , the previous duke is their grandfather. If u ask why their father isn't a next succesor,.... He's just hopeless he's just a mf who threw his seeds everywhere
They don't explain it in detail in the novel, they just said that her father died by an accident and her grandmother followed him due to her grief and even after she died, her grandmother had planned to poison her mother when they were about to be taken to the main mansion and obviously her plan worked, her mother die because of that

So is her grandmother still alive??
And where were her father again??
Is her mother a commoner birth?