ALL circumstances will be taken into account by the jury. The solicitors from each side will also talk to the jury and try to put their points of view across.
It is normal that some witnesses are also related to accused, the two sides solicitors will point this out to the court. It does not stop their evidence from being given. But it could affect how the jury view that evidence.
Forgive me if I am wrong, but I get the impression that your friend feels he should be regarded as not guilty because the victim is a criminal with a long history of violent offences.