Aditi means unbound, and it is built from a negative prefix and diti, a binding. Diti is that binding with the prefix taken off. The relationship between the two names is not a family resemblance; it is a grammatical operation, and it is the first thing anyone notices about her.
What follows from it is a real question about which came first. One reading takes Diti as an old goddess in her own right, the bond that Aditi releases, so that the pair are a genuine opposition inherited from an early stratum. The other takes her as a back formation: the abstract noun was extracted from the divine name to give the negation something to negate, and the goddess was assembled around it afterwards. The evidence favours the second more than the first, because Diti is almost invisible in the Rigveda. She is named a handful of times, usually in the same breath as Aditi and usually in a formula, and she has no hymn, no epithet and no story anywhere in the collection. A figure that old with that little attached to her is unusual.
Her career begins later, and it begins because the genealogists needed her. Once the Puranas organise the population of the cosmos into a single descent from the sage Kashyapa, two co-wives are required to produce two opposed families of powerful beings, and the two names already stand in opposition. Aditi bears the devas. Diti bears the Daityas, whose name is simply her own with a patronymic suffix. Whatever she was in the oldest material, in the literature that most readers meet she is the other mother, and the whole of the deva and asura rivalry runs through the difference between the two households.