Is it considered a sin to inform people about the dissolution of an engagement, even though it hasn't been officially annulled between the families, especially since the families believe the reason is "sihr" (black magic) and refuse to annul it?

Do not rush to break off the engagement with the young man if he is religious and has good character, especially if your family approves of him. If your dislike for him is due to magic (sihr), then efforts should be made to treat it with ruqyah as prescribed by Sharia. However, if the young man is not satisfactory in his religion or character, there is no harm in breaking off the engagement, as it is not binding. If you break off the engagement, there is no harm in informing people that you are not engaged, as this conforms to reality. Otherwise, your statement might fall under the category of lying.