Respectfully snipped, and BBM.
I do agree that Dina is the smarter, more sophisticated, and more educated of the 2 twins-- listening to the 2 interviewed makes that apparent. Nina likes to talk and ramble on. When given enough time, and enough opportunity to talk uninterrupted, she reveals quite a lot in her extended stream of consciousness.
JMO, but I think Nina has lived in Dina's shadow for many years, and may not be entirely content with that. However, I think she is very loyal to her sister. And clearly, they both loved Max, and both hated Rebecca. (Nina's vicarious hatred in support of Dina's, imo.) If there were any nefarious action on Dina's part, Dina may be counting on familial manipulation, and Nina may be agreeable to going along with the plan. Nina may really enjoy the perks of being associated with the wealthy Shacknai lifestyle that Dina married into.
Conspiracies are very difficult to hold together over time.