Explanation:

- Took her for: This phrase means to mistakenly identify someone as someone else. In the context, it means she looks so young that I thought she was your sister.

- Correct Answer: Option 2 - took her for

- Took her in: This means to take someone into your home. Not relevant here as it doesn’t fit the context.

- Took her to: This means to accompany or bring someone to a place. It doesn’t fit the context of mistaken identity.

- Took her over: This typically means to gain control of something or someone, which is not relevant to the sentence.