Hello Anna 66,
At Roxey's age she is becoming a fully mature animal, socially, emotionally and mentally. Being a Sib she will have less of a domestic dog attitude to life and more of a wolf attitude - a bit more wolf than your average pet if you like. It will be perfectly natural for her to improve her own status by noticing any weakness in older animals that she can take advantage of. I think this is exactly what is happening. You can help things along and reduce the stress for Angus by altering the way you handle the dogs. Treat Roxey as though the she is the higher ranking animal of the two. This will allow Roxey to feel more secure and she can ease off Angus. Angus, I'm sure, will breath a large sigh of relief and his life will be a lot more comfortable. One word of caution - be careful to impress on Roxey that you are still higher ranking than her and that you are strong in your position - she will be looking at you next and weighing up her chances!!!