@FanOfMostEverything
In Homestuck, the Dancestors were basically the pre-scratch (basically the characters swapping places with their parental figures) versions of trolls whose genetics were most similiar to the main batch of trolls. For all we know though, the Mane 6 were not created ectolobiologically… I could think of something like this:

Twilight Velvet (T.S. acts like Rose Lalonde, so her mom is actually still her mother.) + Amethyst Star (Only 5 years or so older than Rarity, acts as Ponet s’ daughter\Dinky s’ half-sister, so is not considered a relative at all.) = Twilight Sparkle + Rarity (Dont’cha feel these two could be ectosisters easily?)

Granny Smith (due to her age, considered a “grandmother”. Big Mac and Applebloom are the kids of Granny s’ non-ectobiological offspring, think like Dad Egbert s’ existence.) + Posey (Pegasi do not raise their offspring, and Posey was an Earth Pony who actually lived and died around 1000-1500 years ago. Known as Firefly s’ best friend\almost-girlfriend.) = Applejack + Fluttershy (Fluttershy got Pinkie s’ wings due to an ectobiological semi-mixup.)

Firefly (Was Posey s’ BBF-almost-girlfriend around 1000-1500 years ago, as Pegasi do not raise their kids*) + Surprise (Is a Wonderbolt and around 9-10 years older than Pinkie, and does not act as her relative at all, not just because Pegasi do not raise their kids, but also because Pinkie grew up as an Earth-pony in an unrelated family.) = Rainbow Dash + Pinkie Pie (Pinkie ended up wingsless and gave them to Fluttershy due to an ectobiology mixup.)

*Except if they get to raise an ecto-created unicorn that was adopted by their unicorn ex-husband, and then give birth to said unicorn ex-husband s’ unicorn filly. Case-in-point: Derpy Hooves.