When you see this on the anime of 91 days page saying it is a sequel it isn't really, it just a special that feature three stories in an episode. The first story was the story of the Vanneti family when they were all young and how their mother dies, the second one was Vincent Venetti joining the family through the use of the playing a card game (I don't remember the name of the game though) and showing how intelligent he is. Then the last one was Angelo and Nero where Angelo was helping Nero since he was sick.
This special was more of a flashback than the sequel which I didn't mind since I did like the anime's ending so I kinda didn't want to know what had happened after but the special was not bad.
Story 7/10
Art 8/10
Sound 8/10
Character 8/10
Enjoyment 7/10
Overall 7/10
