Exegesis:

ara martures este ‘so you are witnesses,’ i.e. to the fact that their fathers killed the prophets, or, taking the clause to refer to what follows, to their approval of the deeds of their fathers. The former appears to be slightly preferable. martus also 24.48.

kai suneudokeite tois ergois tōn paterōn humōn ‘and you approve of the deeds of your fathers.’

suneudokeō ‘to agree with,’ ‘to approve of.’

ergon (also 24.19) ‘work,’ ‘deed,’ ‘act.’

hoti autoi … apekteinan autous ‘for they killed them.’ autoi is emphatic.

humeis de oikodomeite ‘you build,’ scil. their memorials. humeis is emphatic.

Translation:

You are witnesses, i.e. you give testimony, you state that you know, or, since the testimony is not by word but by deed, ‘you show/prove that you know,’ or, with transposition of the object of the next verb, ‘you show/prove that you know what your fathers did.’ If one follows the alternative interpretation, testimony and consent or approval go together, and one may say e.g. ‘you testify and consent to (or, you show/prove that you approve of) what your ancestors did.’

Them, i.e. the prophets.

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.