The Star Wars universe has two memorable Rebel Admirals, Raddus and Ackbar. Both are Mon Calamari.
Raddus - Admiral - Mon Calamari - Rogue One
Ackbar - Admiral - Mon Calamari - Return of the Jedi
Are they related?
While we have no explicit confirmation that they are not related, the odds of two random Rebellion Mon Calamari being related aren’t particularly high. We are dealing with an entire species, after all, one that has been seen on multiple planets, and has at least one that they call their own.
In addition, according to Pablo Hidalgo, Ackbar and Raddus hail from different regions of Mon Cala.
Yep, that’s right. Fans of the squid-like species can get excited for more Mon Cal. New variations of the famous aliens will show themselves in the next installment of the Star Wars franchise. These guys will be even more adaptable and might look a little different. As reported by Lucasfilm Story Group’s Pablo Hidalgo, “Rogue One will show us Mon Cal from the colder polar extremes. Ackbar and the other ones we’ve seen so far are more tropical (and colorful).”
Again, this certainly doesn’t preclude Ackbar and Raddus from being related, but it does make it less likely.