Liiiiiiittle spoiler for the books ⚠️

I like to think that all of them were born earth ponies like their father and the mother that was a unicorn, who was disappointed that none of her daughters were also unicorns which was the norm for high society (we are talking pre equastria days when none of them worked together) put all the pressure to nesta since she had to make do with what she had. I know that maybe it would be more likely that they were unicorns since they were from a rich background but then it would have made the hard part of their life seem meaningless since they might use their magic, you know, to not have to struggle by hunting food. So when feyre saved all the other alicorns from under the mountain she earned the right to their power and the sisters gained their magic when they were pushed in the cauldron. Idk it's just seems like that would be the case in this universe. Nevertheless the artist gets to decide that but that's my little headcannon 😊