Positive and negative are irrelevant to the usage of the word still. As used in the OP, "still" has a general meaning of "up to the present moment", and can be used to qualify words and phrases in both positive and negative sentences.
Here are some examples, expanding those in the OP:
- She is here. (Meaning: She is in this place at this time.)
- She is still here. (Meaning: She is here, although we might have expected her to have left by now.)
These are positive examples, but we can do exactly the same pattern with negative examples:
- She is not here. (Meaning: She is not in this place at this time.)
- She is still not here. (Meaning: She is not here, although we might have expected her to have arrived by now.)
Note the difference between sentence 2 and sentence 4. In the first, still points to an expected departure. In the second, it points to an expected arrival. But that is only because the state of affairs is different as between sentences 1 and 3. What this shows is that both sentences can be modified by still, but the implied meaning may vary depending on the context.
- She is not still here. Here the presumed meaning is "She is not (still here)." In other words, grammatically this is the negation of sentence 2. "Is she still here?" "No, she's not still here." This is not wrong, but in reality English speakers would express it in different ways, such as "She's no longer here."