New dog, old dog siblings began fighting

Question
We had a beautiful, well-adjusted Border Collie and kept one of her male puppies. Joe is a skittish BC mix, non-aggressive...never been in a fight or bit anyone. Was dominated by mother. The BC mother died last year and Joe became more skittish and afraid to go outside of the house. We shouldn't of tolerated the behavior, but I guess we felt sorry for him because of the death of the mother. He seemed lonely and became very attached to my husband and I.

We kept in touch with people who adopted Joe's siblings, and recently re-aquired his sister. (They're 7 yoa now). This dog was neglected and left alone for long periods of time. Not very socialized. Liked to escape.
Everything seemed to go OK for several weeks. They slept near each other, chewed bones together, played (seemingly happy). We noticed the sister becoming more aggressive (chomping more, running after Joe more). Joe seemed to not like the running games after a time.  

One day the sister attacked Joe over a big, cow-knuckle bone. I had to pull them apart and they each had a bloody nose tooth-poke. I'm usually home during the day and my husband's home at night, so we keep a close eye. But it happened twice again over their regular food bowl. (One finished first and walked over to the other's bowl). I made a correction and took the sister immediately to lay down for a long period after each instance in another room. She became markedly more submissive to us after this. She still 'bumps' Joe and still tries to get between he and I. I watch for stares, growls and bumping. I keep a water bottle handy.

We have always alpha stanced the dogs, command sit-stays, and go out the door first. We take daily walks to release energy. It seems to calm the sister, but now we're not trusting of her.

Do we still need to do more? What's the best course of action? How can we assure pack stability for these two?  

Answer
For one thing, do not feed them in the same area, but in their own crates. Or stand there and watch them eat and let it be known that you will not tolerate food stealing. This would apply to bones, toys, etc. also.

Working on the people being alpha is the right thing to do, and I would get the bitch into ongoing obedience classes. The problem with most border collies is that they need a job to do. "A tired dog is a happy dog."

In the case of what is going on between the two dogs, I think they are too much at the same level of dominance so they will just have to settle who is alpha themselves or one of them should be placed. If you think it will get really nasty, just muzzle the two dogs and let them have at it until they reach an understanding.