
i would also say its normal,my large breed bitch had first season at 6 and half months and maggie (tiny yorkie)came in and had her first at 9 months my mums large breed dog when i lived at home in my younger days ;-) had her first season at 6 months too.
i know all dogs are different but if your litter sisters are close then maybe they might come in together,a few of my freinds will tell you(had dogs all theyre lives)that sooner or later bitches that live together petty much follow suit with seasons bit like us i suppose.
give me a buzz havent spoke in ages.xxx