Considering how she has killed more people than any demon has, if she ever actually meets the former Demon King, he'd likely force Twilight to abdicate the throne to her instead.
Well, the good new is that they really were his uncles.
The bad news is that his uncles were the ones who exiled his father, Oh....yeah, and he is the "demon" that destroyed on of the previous 'ancient clans."
So....that sucks for them, I guess.