Maybe she is testing you to see what you look like after hearing this sentence, or, for some reason, she didn't become boyfriend and girlfriend with you. I still suggest you stick to it. Don't give up your love for her just because she said "Let's be brother and sister", because you take this relationship seriously. Since you expressed it and the other party didn't refuse it, you should stick to it. You can get in touch with her more and learn more about what she likes in the future. Do less things that make her jealous and unhappy, and treat her as your girlfriend, so that maybe you can get in touch with her or let her tell you the real reason why you were brother and sister at the beginning, so that you are a real love brave, and her secret or the last time I was with you is your record. If you still like her, don't care what brother and sister we are, and go after her bravely! Maybe you'll catch her?

I wish you success and happiness! Come on! ↖(^ω^)↗