I've had a lot of thought about this, and I loved drawing this while I thought about their family. I had a lot of thought of whether or not Dinky and Derpy should be related or daughter/mother… after a while I decided on this.

Daring Doo, Ditzy (Derpy) Doo, and Dinky Doo are all the children of Written Script and Carrot Top. As you can see there's a lot of gray in their family from Script, and also some yellow from Carrot Top. Dinky is the only one with purple in her coat, and Daring has purple in her eyes.
Daring was born first, with Ditzy coming just a few years later. They both took the last name of Doo, after Written Scripts' grandparents (That's also where they got their wing genes). They passed just before Daring was born, and decided to honor them by gifting their children the last name Doo. Daring and Ditzy got along very well, and played a lot of imaginary adventure games in the backyard. Daring discovered her cutie mark during her 6th birthday, when she and her friends were testing their logic in an escape room. Everything clicked in place for her,and they ended up setting a new record for the fastest time. She moved out very quickly however, the discovery of her cutie mark pushing her to explore all of Equestria to perfect her talent.

Ditzy's talent was found out through a swimming trip with Daring, where they dived for undersea treasures. She was able to hold her breath much longer than her sister, and thus her cutie mark appeared. She has the lungs of a whale, but doesn't use her talent often. Only for treasure diving with Daring, but why hold your breath when you can just wear a scuba suit, right? She saw her special talent more as a fun quirk, and decided to take on mail delivery instead to actually earn a living. Maybe with some convincing, she could embrace her talent in the future.

Dinky was born year later, after both their children had moved out. Their parents found the empty house very lonely, and decided that Dinky would be their final foal. (They just didn't want to give up on parenting just yet :>) Dinky was born, and even Daring managed to show up just hours before the birth. Her older sisters adored her, especially Ditzy. Her parents were happy she was a unicorn, but still gave her the last name Doo to match her siblings. Ditzy is proud to show off her cool sister to every pony in town, and Daring will do anything to protect her from any bullies, just as she did Ditzy.