You described a sibling with a female reproductive system, who has been pregnant. This means that your sibling must be a woman.
That you refer to her as your brother rather than your sister implies that she calls herself a man, and that because she calls herself a man you have chosen to do the same.
You are of course free to immerse yourself in the fiction that she is a man, and make self-contradictory statements like "my brother is a man who has been pregnant." But it would be odd to expect others to agree with this, seeing as it is not based in reality.
That you refer to her as your brother rather than your sister implies that she calls herself a man, and that because she calls herself a man you have chosen to do the same.
You are of course free to immerse yourself in the fiction that she is a man, and make self-contradictory statements like "my brother is a man who has been pregnant." But it would be odd to expect others to agree with this, seeing as it is not based in reality.