1. She has a brother.

2. He has a brother.
3. It has a brother.
[Can we use all the expressions? Does "It' refer to "She" or "He"? When 'It' refers to an animal, can we use Sentence 3?]

1 and 2 are fine.

3 would be fine if it's clear that "it" refers to a cat or dog or other animal, I guess.

1 and 2 are good. 3 is wrong.

Yes, all the expressions can be used to refer to a sibling relationship. However, the pronouns in each sentence will refer to different individuals:

1. "She has a brother." - In this sentence, "She" refers to a female individual who has a brother.
2. "He has a brother." - In this sentence, "He" refers to a male individual who has a brother.
3. "It has a brother." - In this sentence, "It" is generally used to refer to an animal or a gender-neutral object. So, if you are talking about an animal, you can use sentence 3 to indicate that the animal has a brother.

Therefore, the pronouns used in these sentences can vary depending on the gender or the object being referred to.