I would look at the doll she requested and price the accessories on the American Girl Web site. The accessories are more affordable than the actual dolls. My daughter could care less about the dolls, but she flipped for the raccoon in a trash can my parents gave her for her birthday. If that is not in your budget then just think about the most appropriate gift you could give your niece. Then if your SIL has the gumption to complain just tell her the truth the dolls are out of your spending comfort zone at the moment. Besides it is the 6 year olds party anyway. If your niece is happy her mother shouldn't care what you gave her.