Little was bullied mercilously by his siblings, Butterfly turned on him after they had been bonded for a couple of months (and it was the humping from Butters that was the cause of any issues between them) and as you also know, he doesn't have a sense of smell, and sight is his primary sense, which will also play a part.
I did a much slower bond when I bonded Little and Butterfly and we had no problems at all- and Little was submissive.
I'd be more than happy to have them here (in due course obviously) and have a go at a slower method (if your sister is comfortable with them going elsewhere, and also could talk me through exactly what Dexter needs).
I'm glad Dexter got very lucky. I hope he heals very quickly and shows no lasting damage from his run in with Little. I'd suggest stopping all head humping in you do try again (sure you've worked that out for yourself though).
i'm sorry it's not gone well