Please help me find!

Ik that’s it’s a Korean manga.
So, the MC was severely mistreated by his family all except for his older or twin sister who sometimes sneaks in food for him.
One day, they have him pose as his sister to get married to the ML of the neighboring clan under the pretext of a peace treaty but really they wanted him to assassinate the ML.
The MC was very tiny even though he was about I think 13-14 at the time (around there) the clan he’s from has a special power where they are immune to poisons or something like that? Later in the story, he gets wrapped up in this silk like cocoon cause he’s getting “older” or shedding or something that has to do with his clans powers I’m not sure.
The ML treats him very nicely cause ya know it’s a little kid, and even proposes they run away together.
Ive looked through the entire Gender Bender tag and I can’t find it maybe I might have missed it TT