
If Lawrence is older than Tia, does that mean:
the mother married marquess,
became a mistress and had Lawrence,
after that had Tia with the marquess ?
How do they know if Lawrence is the emperor's son?
or the mother marries marquess after she becomes a mistress?

I think the brother is older than MC. And the Marquess (father of MC) trade his wife to the emperor to have sapphire mine. And later on, the wife had MC, maybe the Marquess need a heir so MC is born. MC looks like his father, the Marquess though Idk about his brother coz he looks like the mother so idk about his legitimacy but maybe the Marquess is away that is why it is already given that the Emperor is the father of his wife.

You're somewhat right.
The Marquis sold his wife off (whether that was a plan made by her or not is not clear.) And she bore the Emperor an a "possible" male heir. (The son is not the crown prince.)
She later had a daughter(LF) with the Marquis.
SPOILER:
The Marquis is actually dead. He was assassinated

Does anyone know what manga this is from?
http://www.mangago.zone/read-manga/you_re_my_loveprize_in_viewfinder/mh/iur_chapter-85/pg-1/
I love there's season 4, but I'm not ready for the drama
please don't hurt our Bom ┗( T﹏T )┛