Hmm…well she is fixed 8 months old now, I guess she may be reacting to the season. I am doubting hyperthyroid at this time as their has been no change in water or food intake, and she is still gaining weight, as well her coat is not dulling, just her first coat blow. I am wondering if it may be that she is just finaly trying to take the full alpha position from Charlie, She is finaly taller than he is now, and he was a little mean at times when she was very young. I will keep an eye on her water and food intake, as well as her weight. If she starts intaking more, and looses any weight I will take her in.
One other thing...it appears to be only aggression towards Charlie (Fixed male Parson Russell), not to people or the other dog (A fixed male min-pin)