I have 3 year old littermates Micki and Mini. Things have always worked out fine, Mini is more dominant and Micki has always been submissive. I do not know what we were thinking taking them both and I don't really think I would ever do it again. When they were little it was twice the work, twice the chewing, twice the pottying in the house, twice the money etc. We have never had any aggression issues with the two of them and I am not looking forward to them growing old together. I think its different with brother and sister vs two pups of the same sex. I would never have gotten two female littermates or two males, I wish you luck in whatever you decide to do and we definately need some pictures