Could you explain me why it is better to say
I did not know you had a brother.
even it is a present situation. Your brother is still alive. It is better to match the tenses and why can you use present perfect in this case
I did not know you have been ill.
and not past simple as in the first example. In both cases it may be a present situation (you still have a brother and you are still ill) but the tenses chosen are different. I thought it was better to match the tenses in both cases.
Will it be better to say
I did not know you were ill.
to use past simple and match the tenses as in the first example even if it is a present situation