Explanation:

- My sister prefers: This is the subject and verb phrase. It is grammatically correct and needs no change.

- Tea than coffee: The error lies here. The correct phrase should be "tea to coffee" instead of "tea than coffee." The word "prefers" is followed by "to" when comparing two options.

- Correct Answer: tea than coffee

- In the afternoon: This phrase refers to the time of day. It is used correctly in the sentence.

- After lunch: This indicates the sequential action. It is correctly placed and adds to the sentence's clarity.