A theory that is fairly popular about why Ursula used to live in the palace before she was banished was because she was Triton's sister. It's not very likely, but I thought I'd have some fun with it. If this was true, I'm guessing that Ursula and Morgana were turned into cecelias (octopus ladies) when they were banished for using black magic to overthrow their brother (Although both did it separately and not together). This is what I think they would've looked like as young and beautiful mermaids.