The rules don't mention the possibility of a familiar also being a cohort due to the master having taken the feat Leadership (Player's Handbook 97). However, the Dungeon Master's Guide on Paladin Cohort Mounts provides rules that can be massaged to fit:
At the DM’s option, she may allow a paladin or other character with a special mount class feature to combine the special mount with the cohort feat [n.b. in context, the feat Leadership]. The special qualities such as the empathic link and shared spells make this quite potent and worth a minimum of a +2 level adjustment to the cohort mount ECL. (200)
The typical coure (Book of Exalted Deeds 168-9) has a cohort level adjustment of +5 for an effective character level (ECL) of 7. Using the above Paladin Cohort Mounts as a guideline—and assuming that the DM rules that the special abilities a wizard grants his familiar are as "potent" as the special abilities a paladin grants her special mount—this makes the cohort familiar coure ECL 9, and such a cohort familiar becomes a possibility for any creature with the appropriate feats and a leadership score of at least 13.
The only real sticking point after that is the creature's Hit Dice and hp. Were a player to come to me with this proposal, I think I'd use the master's effective Hit Dice for computing spell effects (as per Familiars on Hit Dice on PH 52) plus whatever Hit Dice the creature gains naturally due to its own advancement beyond this then follow a similar procedure for the creature's hit points—half the master's hp plus whatever hp are gained from advancement. This will make the familiar's effective Hit Dice higher than the master's—and that's weird—, but since the creature doesn't really benefit all that much from it, that shouldn't be a big deal.
If the DM won't adapt the Paladin Cohort Mounts rules to familiars, then it looks like the wizard is stuck with having two sprites, one a familiar, the other a cohort from the feat Leadership.
