Must a person have given a warning in order to be a witness? What if 2 people (1 of them the brother of the killer) witnessed a murder after giving a warning but there are 2 other witnesses (not related to anyone else involved) who did not give any warnings? Can the two who did not give warnings testify in court? (The two who gave warnings are disqualified.)
Yisroel K, Beit Shemesh
The alleged murderer must be warned in order to be punished with death. The warning does not have to be done by either of the witnesses of the murder, but there must be evidence; two witnesses who heard the warning (the two can include the warner himself).
So if he was warned even by a disqualified person, and there are two valid witnesses on the warning and on the act of murder, then Beis Din can give the death verdict.
All the best,
Reuven Weiner