Now, say, I am talking to A about B's brother:
-He is/was a man of bad habits.
In English grammar, usually when referring to a dead person, or things related to him, we need the past tenses, but when I don't know whether the person in question is dead, like B's brother, what tense would native speakers use?
The general rule of thumb is to refer to a specific time period when describing habits. If you are talking about someone's childhood habits you would say:
Alice was a girl of bad habits.
Likewise if they have been dead for some time:
Gandhi was a man of bad habits.
If they have recently died then there is no proper way. Follow the pattern used by the grieving family if possible; otherwise I would recommend past tense.
But as for your particular scenario, where you don't know if the brother is alive or dead, refer back to the time period being described. If you are talking about how he acted back in school use "was". If you are referring to their current or recent habits use "is".
As far as politeness, it is significantly less offensive to assume someone is alive than assume they are dead. When in doubt, consider them alive until otherwise informed.
