You're not dumb. But it's an ambiguous question as written. Presumably, we are to assume that all siblings involved are still alive, that all all full siblings (ie no half- or step-siblings), that everyone is cisgender, and that Julia is female. In which case, Julia would have one sister. Each of her three brothers would have two sisters, one of whom was Julia herself.
My 6yo can answer this correctly, making all of your stated assumptions above. Edit: Clarification - I am in no way implying that someone who was confused by the math is dumb, everyone makes silly mistakes sometimes.
I think It’s only ambiguous to a machine. To a human, even in 2023, these are likely default assumptions for a question phrased as above, unless being overly pedantic or attempting to “get around” the puzzle.
If you ask a model - or a person - “what does 1+1 equal to?”,
it is also ambiguous, since we haven’t defined what “+” means, did not specify that we are dealing with natural numbers rather than eg elements in Z//2Z which could be similarly denoted.
