... ... ...I will choose to see your sentence as a ''neutral question''.
If I would have to put the main theories into %... it would be something like : 80%-Born from Derpy/Ditzy (see the 'hereditary genes' to explain why Dinky is a Unicorn, unlike her Pegasus mother) and 20%-Adopted by Derpy/Ditzy.
Overall, from the looks of it, most people appreciate more the idea of them being directly linked(for a lack of better word).
Huh... just type 'Dinky Derpy' or 'Dinky Ditzy' right here on DA ? It's a start. [link] [link]
The story archiver thingy, at Eq.Daily, also allows to filter only the stories that have Derpy/Ditzy in them, though you can't make it more precise (Dinky isn't in the filter-keywords), but that's still something : (about 101 stories at the moment, if you feel like digging) [link]