Andariel's blood relation to Mephisto and Lilith makes it hard to tell what actually makes a demon a Lesser Evil. Lilith, Andariel's mother, Mephisto's daughter and a very powerful demon, is not considered a Lesser Evil. If so, the title of Lesser Evil has nothing to do with blood ties that an evil may share with other evils, like the Three brothers are.
Perhaps the status is simply conferred to the demons of greatest strength. If so however, the number could be changing, and Lesser Evils overthrown. It may also be related to areas of rule in Hell. The manual of Diablo I reveals that the Lesser Evils rule supreme in their own domains in Hell. Perhaps the status of Lesser Evil is then conveyed to whomever possesses such a region. This would also bring with it the conclusion that the title of Lesser Evil can change. It would also explain why Lilith isn't one, seeing as she's an outcast and rogue demon.