“Why can’t you be more like your sister?” has no right answer. Don’t try to answer it with your whole life.

That question is not really a question; it is frustration arriving in the shape of one. No answer is available, because what is being asked for is that you be somebody else, and trying to satisfy it reshapes your choices around a comparison you never agreed to. People sometimes say it without hearing what they are asking.

You can let it pass without taking the assignment. If it keeps coming, you might say once, calmly, that you are not her and would rather be measured on your own terms. Then decide by what you actually want, and check whether a plan is yours or a reply.