I have 2 female litter mates that are 13 weeks today and they also look completely different. My bigger female looks like most Yorkies and at 11 weeks weighed 2 lb 8 oz and my small female looks like a tiny fox at 1 lb 14 oz. I took their measurements and they aren't really that different except in head size and length. My big girl (Rosie) is 2 1/2 inches longer than my small one (Bella).
Yorkies are like kids, sometimes sibblings don't look anything alike!