@haru_bun as far as she knew he got sick getting her a birthday present. now we know that her father's first wife had him killed(admittedly by directing him through a town that had the plague) odds are the eldest was born of the mistress(Tara's mom) as the first male heir that would typically make him the one to inherit the title in most noble families, odds are she wanted one of her kids to inherit the family instead and tried to remove him.
this whole incident sparked the problem where the original Tara locked herself in her room and refused to leave(why she gained weight) at the beginning of the story which the reincarnation resolved, now she's depressed again.
@ginger89 he was only ever mentioned, but it was at the beginning, it's why she was fat at the start(OG Tara i mean), she locked herself in her room and ate sweets because she didn't want to do anything after losing him because she blamed herself for his death, it's also why her siblings were so hostile at the start of the series.