Okay, I might be going on a stretch here.
Last year I figured out Mal's father is probably the King of Hearts. Now, we don't get to see a lot of Ursula in Descendants 2, but we do see that she is still an octopus and not human, while Uma is. This means her father must be a human. We also know he must be a villain, as Uma was born and conceived on the Isle and only villains live there. Now let's get into the genetics. Again, we don't get to see Ursula a lot, but we can assume she is black in the Descendants universe, as she is portrayed by Whoopi Goldberg (Just like how Wendy Raquel Robinson portrayed Cruella de Vil). Uma is also black, and not just tanned so her father can't be white.
A dark skinned Disney villain... DOCTOR FACILIER!
If you need any more proof, look how similair Freddie and Uma are. Freddie is also voiced by Lauryn McClain, the sister of China Anne McClain (Uma's portrayer). It could be very plausible Disney is hinting Uma and Freddie are actually half-sisters. Now for more proof: Freddie has grey streaks in her hair on a young age, Ursula has grey hair...
https://descendantstheory.blogspot.com/2017/08/descendants-theory-who-is-umas-father.html
