we only know for sure of the 3 you listed. steele is rumored to be one but the only evidence of the rumor is her npc skin, for all we know she could have just painted the designs on herself. IF steele is one that leaves 2, which i highly doubt there will be any in dlc because they could be the sirens in the next 2 borderlands games.
"My judo throws are the best, the best throws in the world. All the other throws are done by little girls"-Karo Parisyan
Steele and Angel don't count, "having" been a siren (or speculated possibility) is not a siren. As of now the only ones we know of are Lilith and Maya. Angel and Steele are both dead meaning the siren gene has been passed on.
Now I'm not sure as to how the lore of the Siren's work but when one dies the next siren is it straight from birth or does the "siren" gene/ability/heritage w/e you want to call it have to pass on to a newborn or can it pass on to anyone?
I can't remember if it truly explained that outside of "Only 6 exist at a time" type deal.
There might be another siren because angel died, we don't know how one becomes a siren (born or just one day poof you're a siren). We also don't know if there's an interval between siren awakenings, Though I imagine there is seeing as how the three we know of are all ruffly the same age.
Winner of the Official hacked hurt/heal contest Angel of life
Well, you can remove Angel from the last list post game
The Siren that would have replaced Steele would be 5 years old in the events of Borderlands 2, the one that replaces Angel would be just born/unborn (depending if a Siren is made at the point of conception, or the point of birth).
Plus the Echo's are recorded before the death of Angel, so those 3 would be the 3 that Jack knew of.