Answer:
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Sia didn't marry anyone... The novel ended with her travelling to the mountains with Chunho (which wasn't in the manhwa,, idk if he'll ever be in the manhwa since he showed up before Sehwa announced that she can cure Sia) and Lee Sihye (well, Crown Prince Seungwon, the dead brother, which ig the big thing that happened).
MORE INFO abt late Crown Prince/Lee Hye/Lee Sihye being alive: He faked his dead. Reason? Related to Depose Lady Hong and her son, so they wouldn't get more power and such.
omg i knew it. they never actually confirmed it was his body but a mangled corpse wearing his attire. as more time went on and nothing regarding the former crown prince was really hinted at i began doubting myself. i personally thought it was because he couldn’t handle the stress of crown prince and knew his younger brother was far more suitable and more deserving.
That too but that wicked consort and prince was more of the reason. Many princes and princesses were killed because of them and their maternal family wanting power. The only reason the Current Crown Prince is alive was that his mother, the current Empress, hid her pregnancy till she was able to give birth safely. But it was also theorized that she and the Current Crown Prince (when he was young) was sickly because of the attempts.
I read here: https://novelnuku.com/series/born-as-the-daughter-of-a-lowly-concubine-en/
I suggest you read from the start tho

The manhwa doesn't strictly follow the novel so ask away