GPT4 gets the second question wrong for me, but in a weird way.
> Julia has three brothers, each of them has two sisters. How many sisters does Julia have?
Julia has two sisters. Each brother having two sisters refers to the same two sisters, Julia and another sister.
Then if I ask to verify its answer, it realizes the issue.
> Apologies for any confusion. If Julia has three brothers and each of them has two sisters, then Julia is one of the sisters. The other sister is common among all three brothers. Therefore, Julia has only one sister.
> Julia has three brothers, each of them has two sisters. How many sisters does Julia have?
Julia has two sisters. Each brother having two sisters refers to the same two sisters, Julia and another sister.
Then if I ask to verify its answer, it realizes the issue.
> Apologies for any confusion. If Julia has three brothers and each of them has two sisters, then Julia is one of the sisters. The other sister is common among all three brothers. Therefore, Julia has only one sister.