Yeah, a bit late for that. The Lombardy have been eyeing her like a snake for a while now (though Bellesac's branch remains clueless and thinks becoming the Emperor's servants is a good thing).
I would like you to remember that the prince survived being starved and poisoned for YEARS before overthrowing his brother in the previous timeline. Tia did what she could to help by providing him with an antidote, but there is realistically no way for her to interfere without her or another member of her family regularly entering the palace--which any Lombardy with sense will not want to do.
And I don't remember if this happened in the webtoon yet or not, but in the novel Tia does tell her father about him (she says she made a friend at the palace). However, he is no more able to solve the situation than Tia is.
Ok soooo maybe I missed something wont be surprised that I did but um she knows that that black haired boy is being poisoned and starved and she doesn't tell her dad or granddad??? That's so fucked up