Bad move to bring out food. Food is a very important resource for dogs. Even though you were sharing treats, you were giving out food on Darby's territory, so in a dog's mind, Darby's treats. He went for the youngest and the last one introduced. Dogs will generally target whichever dog they percieve as the 'weakest' when they want to make a point.
Luckily dogs are very forgiving, and you can try introducing them again and they will probably have forgotten the incident. Just dont bring out food for the dogs and if you are going to give the dogs food, seperate them. Its also better not to introduce toys either (or anything else the dogs value)
On xmas day at my brother's home for lunch my mum gave the dogs a bag of treats (I did tell her not to). Now my brother's dog was so happy to see my pups until the treats came out - and then he pinched the lot of them. My dogs, being older, deferred to Jaspar on his turf and let him take all the treats - after all, Deska does the exact same thing to other dogs when they are in his yard (he will even bite other dogs). Needless to say, I went out and removed all the treats.
You are talking about adolescent dogs who are still working out doggy ettiquette, so Darby's over the top reaction is not unusual.
PS - ask your SIL if he is always bringing out resources (treats or toys) for willow when other dogs are around, might be why she is being attacked.
Sorry about the long post - post-xmas brain fatigue.
In hindsight, the treats were a bad idea. I think you might be hitting it close to what motivated the whole thing. Darby and Deja seem to be doing fine now. I'm filing this away for future reference though. Bite me once, shame on you.. bite me twice, shame on me. (metaphoriclly speaking hehe)
Is Willow eating alot of liver?