It seemed about time to update Destiny's ref and let you know that yes she is a twin, nearly identical
This is Destiny and Daylight at age 6 and a half,
No visible differences
Symetrical
Yes those are spots around the heart on their flanks I know they are tiny and hard to see
Both are the same size, which is slightly smaller than silvey who is one years old and pretty normal in size

Info: Only two ponies can tell them apart, one is Silvermist, or Silvey at this point who is one and a half years old as seen on her own ref
They are not alike in personality but similar enough to be easily mistaken for one another, especially because they both enjoy causing chaos by pretending either to be each other or that there is only one of them. Neither has any magic ability on purpose alone, but by crossing their horns and trying the same spell together they can do a little minor magic, such as light weight levitation and flicking switches.
They are mischievous brats and proud of it
They are very strongly connected and can actually communicate at a distance with no words and feel each others feelings
Both are more defensive over each other (and their friend silvey) than themselves
Both are equally happy playing with bugs https://40.media.tumblr.com/a273d63.....o1_r1_1280.jpg as dressing in frilly stuff and playing house
I have so much more info but I will save the rest to be learned in roleplay