I say "old" because this AU was just made for fun and I didn't really plan on expanding it so it kind of got abandoned
Anywho,
This AU focuses on Princess Sugar Heart, or just Sugar, the daughter of Princess Flurry Heart and Pound Cake, 

and Dark Decade and Twilight Century, offspring of Princess Twilight Sparkle and Flash Sentry.

Sugar was born in the Crystal Empire, but was later moved to live with her father in Ponyville. However, she wanted nothing more but to be like her grandfather Shining Armor, leading the Royal Guard and protecting Equestria from danger. Once she was old enough, Sugar would have to begin her training in Canterlot. That was also where she would meet her cousins for the first time.
Dark Decade (born with large wings) had already been training in the Royal Guard, and was pretty surprised to see his cousin rise in the ranks so quickly. Twilight Century, on the other hoof, was always anxious and difficult to talk to, so she couldn't really get along with Sugar. 
The truth behind these siblings is that they grew up pretty distant from their parents, for the Princess of Friendship and Captain of the Royal Guard were too busy with their duties to care for them. Despite hopping from caregiver to caregiver, Decade and Twilight usually cared for each other themselves. Decade eventually joined the Royal Guard to try and get closer to his father, while Twilight continued to lock herself in her room.

Desperate to socialize her cousin, Sugar would drag Twilight out of her room and practically force her to hang out. But no matter where they went or what Sugar tried to talk about, her cousin would either barely speak or complain. Over Time, Sugar would get so frustrated with Twilight's difficulty that she gradually expressed her frustrations on her.

And this is pretty much where this AU abruptly ends. I haven't written anything further for this, and it became abandoned  However, I'm still keeping these OCs because I like how their designs turned out.
Maybe one day I'll make a Next Gen AU that I like and will continue, but today is not that day