Yorkietalkjilly always gives the best advice.
I should add that an 11 month old unspayed yorkie could simply be coming into her own and maturing. They are terriers, after all, and while most yorkies tend to get along with their housemates... terriers in general are more prone to dog aggression than some, specifically same-sex aggression. Sometimes it's not necessarily a behavioral issue but just maturity and drive kicking in.