I think you've already been given some good advice and I'm not sure I could really add anything. I'm thinking the reasoning is like Aly said, Charlie's place in the pack is feeling threatened, and perhaps Anna is lowest on the rung so she is getting all the heat. I once watched a show on a wolf pack and rather than focus on the alpha wolf, it focused on the omega wolf, which is the lowest ranked wolf in the pack. Every pack member tended to pick on and take out their agression on the omega member, the poor thing was the pack scapegoat. But it happened later that particular omega got killed, injured in a hunt I believe, and the entire pack actually mourned her for a week. They didn't play or hunt and they just laid around, subdued.

Trying not to get off track here, but I'm thinking if Anna is taking the role of the omega member of the pack, that might also explain some of Charlie's aggression. He could be taking out any and all frustration he may be experiencing on her. I agree talking with a behaviorist might be a good idea, to get some one on one ideas at least.

I remember when Tasha was 6 months old and suddenly began lunging/growling at strangers near her, I was alarmed and talked to my vet about it who recommended a consultation with a behaviorist. This particular behaviorist charged nothing for a first consultation and he came to my house to see her and her reactions for himself and to talk with us. He took about an hour or so with us, meeting Tasha, gauging her reactions as he stood near her, then sat down, then laid on the floor (as soon as he laid down on the floor, the most non-threatening position, she came over to him wagging her tail). I was always impressed at how kind, insightful, and willing to do as much as he did free of charge, but I don't know how common it is to find people like that.

Best of luck though.. It would indeed be tragic for any pups to have to be rehomed due to this. I think it is still early enough to be able to keep it from escalating.