Is there a sin incurred by one who lies about a dream vision with the intention of compelling her sister to wear the hijab?

Lying is a reprehensible trait that is not needed in calling people to Allah. Allah has clarified the ways of Da'wah (inviting to Islam) through wisdom, good counsel, and disputing in the best manner. Lying is neither wisdom, nor good counsel, nor the best manner of disputation.

Lying is one of the major sins. The Prophet, peace and blessings be upon him, said: "Beware of lying, for lying leads to immorality, and immorality leads to the Hellfire. A man continues to lie and strives in lying until he is written before Allah as a great liar."

One must beware of lying. Your friend should pursue legitimate methods in inviting to Hijab, through encouragement and deterrence, and not by fabricating visions and dreams.