1、妹妹如果黑了,按照现行法律以及相关案例,哥哥或者父母将来还能通过亲属移民的性质帮她黑转白么?
-No because she is or will be > 21 when your parents become US citizens. You (brother) can not帮她黑转白
2、妹妹的探亲签证,可否在美国境内转成学生签证呢?
-Yes.
如果可以的话,转签成功概率是不是比较低?
-Normally it should be fine, but I still suggest her to get the F1 visa in China if you do not or before you file I-130 for her. If you file I-130 for her, she can be considered to have immigtration intent and thus, it may be difficult for her to apply for her to apply for the F1 visa.