There are two ways of saying it as a statement. They can both be turned into a question. In the statement, the word order is also different:
I bought a present for my sister.
I bought my sister a present.
When you turn the direct object into a question word, it goes to the front, so the only word order difference between the questions is the word 'for', but really they still have their different structures:
What did you buy __ for your sister?
What did you buy your sister __?