I have a theory that relationship effects the quality of portraits. The previous portraits of Dina were done when she and Nina were enemies. I think Nina with the childish trait painted an ugly pic because she was mad at her sister.
Currently, Nina's relationship with her sister is sort of neutral and I think the portrait reflects that. Obviously, she has a better relationship with her husband and his portrait is fairly clear. To test the theory, I had her paint portraits of the twins. As you can see, Mia who has best friend status has a much nicer pic than Ava who is only a acquaintance.