The woman had no doubts that the answer could be anything except 50.
That was her final answer because the question says “half the age” of the big brother. She simple misunderstood the words as is the case with most riddles.
“How is it not 50?” was her repeated question after the man told her the answer was wrong.
“You said the little brother is half the big brother’s age,” she continued with her calculations.
“So when the big brother is 100 he’s going to be half 100 which is 50!”.
After the man explained it out to her, she ended up admitting her failure and laughing it off, “OK, I get it. Hahaha!”