5. still hasn't decided what country she wants to visit
My sister, Lucy, still hasn't decided what country she wants to visit.
"Still" is just saying that it's been a state that has existed longer than expected, even up to the present.
In (3) and (4), the use of "where" is extraneous.
(3)...is undecided about
the country [where] she wants to travel
(4)hasn't decided which
country [where] she wants to travel
Since (3) and (4) already use the words "about" and "which", we do not need to add "where".
You'll notice that for (5), there is no extraneous word added.
"still hasn't decided what country she wants to visit"
"what country she wants to visit" - is all 1 phrase
"what country where she wants to visit" - this would be incorrect
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