You original question in 2019 was why "I did not know you had a brother" is considered "better" than a construction that uses the present tense like "I did not know you have a brother". I'm not certain the answers provided before entirely address the 'why', and some comments are unclear about whether it is correct at all.
In short, it isn't better. It is just one of several equally valid ways to express the same idea.
Others may disagree, but it boils down to whether you take an extremely prescriptive view of grammar or not. The traditional 'rule' would suggest that the tenses you use in a sentence must follow a logical sequence reflecting when things happened in time.
I did not know (in the past) that (at that moment in the past) you had a brother.
It is logically impossible for you to know, for certain, that the brother will still exist in the future, therefore your tenses should, it might be argued, also refer to the brother as if he exists in the past.
Nevertheless, I suspect only an overly strict grammarian would pick you up on using the present tense instead. Most native speakers would find it impossible to find anything wrong with the following.
I did not know you have a brother.
Take the following example:
The train leaves in 5 minutes. I didn't know the train left so soon!
The train leaving is in the future, but even so, because it was a moment in the past when I was ignorant of the time, I can - some might say should - say 'the train left' or another construction of tenses that follows a logical path in time. But you can use a variety of tenses, all equally validly:
I didn't know the train was going to leave soon.
I didn't know the train would leave so soon.
I didn't know the train leaves so soon.
I didn't know the train was leaving so soon.
I didn't know the train is leaving so soon.
In this context, it makes no difference which one you use, and I don't think there is any good reason to think one option is better than another.
The only thing you can't say is:
I didn't know the train had left so soon.
as that places the moment the train leaves further back in the time than the moment when realisation dawns of the train's imminent future departure.
Similarly, the tense of 'have/had a brother' really doesn't matter. Nevertheless, it does illustrate how 'correct' grammar can sometimes introduce ambiguities that 'incorrect' grammar avoids. If you say 'had a brother' it does introduce ambiguity about whether the brother still exists - you could accidentally imply he was dead. Usually you won't, but you could. Saying "I did not know you have a brother" avoids the problem.
There is a lot more on this problem at Wikipedia.