One of the laws of a witness who presents evidence in a capital crime, is that if he is caught lying under certain circumstances, he is subject to the punishment he attempted to implicate the innocent man of:
וַעֲשִׂיתֶם לוֹ כַּאֲשֶׁר זָמַם לַעֲשׂוֹת לְאָחִיו וּבִעַרְתָּ הָרָע מִקִּרְבֶּךָ – You shall do to him as he plotted to do to his brother. (19:19)
Rashi notes that the Gemara in Makos deduces that this only occurs if the liar is caught before his plot succeeds, and the innocent man has not yet been framed and killed. The underlying assumption is that the word ‘brother’ implies the innocent man still lives, as ‘brotherhood’ refers to living people.
The Ritva queries out that Yibum references brotherhood, and can only exist when a brother has died, and that Nadav and Avihu are also referred to as brothers after their deaths.
Clearly the answer lies in the definition of brotherhood. What is the difference?
R’ Ezriel Hildesheimer explains that there is a difference between a biological brother and a fraternal brotherhood. A biological brother remains so after death – the relationship is in the blood. It then makes sense for the Torah to refer Yibum and Ahron’s sons as brothers after death.
However, witnesses intrinsically cannot have any blood relationships to people they testify about, as a condition of testimony. The brotherhood then, can only mean the ideological kind – they are brothers in being bound to observe the Torah, the fraternity of the Jewish people. However, once deceased, they are free from the mitzvos – there is nothing binding them as brothers.
This can only mean that כַּאֲשֶׁר זָמַם לַעֲשׂוֹת לְאָחִיו refers to a still living person!