My Little Pony: Friendship is Magic - And she got demoted to foal sitter after the insanity that ensued.

Or that she's the child…

Bottom line: Cadence's power works EXACTLY the same as the Love Potion from Hearts and Hooves Day – two broken heart bubbles come together with a distinctive sound FX, which was the same in H&HD and Canterlot Wedding. There has GOT to be a connection between the two, although I strongly reject the notion that Princess Heart and Princess Cadence are one and the same.

We know that Prince Blueblood and Princess Cadence are Celestia's nephew and niece, 52X removed (per Word of God), and we know that Princess Luna is most likely single and childless like her sister (and was out of action for 1000 years), so logic dictates that there was another Royal sibling who was ancestor to the mortal royals. Princess Heart is thus the most likely candidate to be that missing sibling.