Answer:
The answer is probably either B or D
Explanation:
It's hard to give a correct answer without more context. Depending on the context of the story, it really could be any of these. However, it sounds like the speaker's comparisons to her sister are more negative than positive, drawing from the other answers. I would read into the text a little more, and see what kinds of feelings you can identify from your speaker towards her sister. This should give you an idea of why exactly she compares herself to her sister all the time.
I hope this was somewhat helpful!