My mother probably never studied such theories in HS back in the late 1930s. I think she was most going on the old wives tales and assumptions at the time that a dark haired and brown eyed woman was not going to have a blonde haired and blue eyed child.
What is the birth order position of your blonde niece?
She was their second child. She was born 5 years after her older sister and less than two years before her younger brother.
I would have bet on the fact that your blonde niece wasn't the first born ( thank's Mendel...your theories still work! ) and I was right.
I do well, re members of my own family, because I can go back many generations. With others, I can only do the most basic of figuring; yet it does still hold up. :-)
Look at your own family, now, and see if you can make a chart of who had what hair & eye color and if it adds up. It's fun. :-)