Dinky Doo is Ditzy Doo (nicknamed Derpy) only foal from an affair she had with an older stallion from Canterlot, named Crystal Lake. It was unknown to her at the time the stallion was already married with a foal. Finding out about the family and her own pregnancy, she decided to keep quiet and raise the child on her own.
Years down the line Crystal's "adventures" caught on with him, making Blue Belle move into Ponyville with her two foals, Amethyst Star and Liza Doolots. Unknown to her that one of the mares lived there. The two fillies still visited their father time to time.
Amethyst few years later found out about the fact that Dinky is her half-sister and decided to be part of her life as one. The while also getting to know Derpy too.
Additional stuff:
- Derpy and Amethyst look the same age, but there's an around 8 years difference between the two (in the beginning of the show's timeline Derpy was 28 while Amethyst was 20).
- I said Crystal Lake was an 'older stallion', he was in his early 30s, while Derpy was 21.
- Like a year before the first Sisterhooves Social we saw in the show, was when Amethyst found about her family ties with Dinky and during the event's time Liza was visiting their father hence why she participated with Dinky in it.
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So I thought a lot about how to do the parents. Should I make up 100% original characters or use characters from the show? In the end I went in a third direction. The show many times based characters names or/and looks on old generation ponies. So I decided to do the same.
Crystal Lake's colors and cutie mark based on a G3 character with the same name, while Blue Belle based on a G1 character. Both characters were originally earth ponies (and mares also, but 99% of ponies in old gens were that, so that's not suprising XD).
Also, I must say, time to time headcanons can be super difficult to figure out how to do. Because how background characters are used, you kinda have to ignore some stuff while others you don't. It's fun but really challenging.
This one turned a bit messy tho.'''''
Also much love to the website, Strawberry Reef. It's a database for My Little Pony toys from all generations. It was super helpful creating the parents.