Yes, PLEASE DO NOT let mom near EITHER Borck or Ollie but especially Ollie. I was devastated when I saw that she had Ollie back because I felt certain that he would die. It is definitely not a good idea to try to get her to accept him or nurse him again. She has definitely decided to abandon him because she thinks he won't survive and is a danger to the others. She may think this about Ollie as well; I would NOT risk it. If you want to get her by the other 2 kittens that is probably best but at this point I'd make the decision to permanently separate Brock and Ollie from her and raise them together away from her. It will mean more work for you but it is definitely their best chance of making it through this.