Don’t capitalize the before Twin Dreamers.
Are there other sets of Twin Dreamers who don’t have to stay asleep? If not, add a comma before who.
Add a comma before claiming. (If you leave out that comma, you’re saying the sister is the one making the claim.)
There’s a typo in the next-to-last line. Change sill to will.
Get rid of the quotation marks. (They’re in a weird place anyway. Why not include will inside the quotes, if you absolutely insist on having them?)
You are a guardian of the Twin Dreamers, who must stay asleep for eternity in order for the universe to remain stable. One day, one of them wakes up and flees with his still-slumbering sister, claiming he will not be a prisoner anymore.
Consider rewording the end of the first sentence: to maintain the stability of the universe. I also think claiming ought to be replaced with declaring.