Is lying in a dream for a righteous reason—such as averting harm from one's brother—considered a sin for which one is held accountable?

What you did is a forbidden lie, and it is one of the major sins. Whoever dreams a dream he did not see is tasked to tie two barley grains together, and he will never do so. This is a metaphor for the continuous torment on the Day of Judgment. The one who lies about a dream is lying about Allah, as he claims that Allah showed him what he did not see, and this is a greater sin than lying about people. Therefore, you must repent to Allah, and there is no need to inform your brother. It would have been more appropriate to resort to permissible means of giving advice.