There is no reason why your mother of the bride dress could not be something that can be worn again for another formal occasion. This piece is an investment and something that you should be able to wear over and over again.
Odds are that the current mother of the bride would not remember what was worn over 5 years ago. Since I am sure this dress is a wonderful piece and something that has looked great when worn, no reason why this could not be used for the black tie invite.
Have a wonderful time at the event! Please when you have a moment to take a minute and rate my answer so that I know that my answer was helpful as well as allowing me to assist other customers.