The “maid in jail” is the sister (=・ω・=)
He's using the sister as leverage because the crown prince is willing to do anything for Euhnas. The only thing Euhnas wants is to run away with her sister but in order to do that the sister has to be freed first.
Are you sure??.. I specifically remember that young master who is "in love" with Enuha saying to the sister that they did not have any choice in the matter by getting married. Then he had sex with her and being able to do and wanting to do it because she looks similar to her younger sister.
Why is the prime minister making a big deal about his son marry Euhnas sister? And who is the maid in jail? How does this play into the plot?