(וַעֲשִׂיתֶם לוֹ כַּאֲשֶׁר זָמַם לַעֲשׂוֹת לְאָחִיו וּבִעַרְתָּ הָרָע מִקִּרְבֶּךָ’ (דברים יט:יט
‘And you shall do to him like he plotted to do to his brother’.
Rashi explains this pasuk by saying ‘The pasuk says ‘like he plotted’ and not if he actually did [to his brother].’ The Mishna in Makos 5b learns this דין from the word ‘brother’ since ‘brotherhood’ only refers to live siblings.
The Ritva asks, ‘When it talks about יבום in דברים כ’ה:ז we find a reference to brotherhood despite the brother having deceased, thus contradicting the Mishna above. For example, despite their tragic death, Nadav and Avihu are also referred to as ‘brothers’.
R’ Ezriel Hildesheimer answers with a most wonderful idea. He explains the difference between a blood brother and a friendship style brother. A blood brother will remain a brother even after he has deceased, hence the relevance of Yibum where the pasuk refers to them as brothers. Nadav, Avihu, Mishael and Elitzafon were blood cousins which hold the same importance, in their case only, as brothers. However, here we are discussing witnesses where they may not have any blood relativity at all. In this pasuk, the reason they are addressed as brothers is simply because they are ‘brothers in arms,’ I.e. they are both keeping the mitzvos. As is proven in Bava Kama 88a, ‘A Canaanite servant is a brother to us in mitzvos.’
This question over whether it is truly a blood brother or just a brother through mitzvos is only relevant if the ‘brother’ is still alive. We have already established that it refers to ‘brothers in mitzvos.’ However, once they are deceased they are free from the mitzvos so there is nothing binding them as brothers.
In conclusion, the posuk is cryptically trying to show us that, indeed, the victim in question is still alive and was not affected / killed by the ‘plot’ against him.