BDAG states that the semantic range of εντος includes 'in', 'in the midst of', 'among', and gives references from LXX and extra-biblical sources that demonstrate the variety of meaning.
Thus (as other answers have noted), whether it should be rendered "The Kingdom of God is in you" or "The Kingdom of God is in your midst" cannot be determined solely by the semantic range of εντος.
BDAG suggests "in your midst" or "among you" as the most suitable translation and states:
Luke generally avoids reference to God’s reign as a psychological
In fact, the Bible as a whole does not primarily refer to the Kingdom of God as merely a pyschological reality, but the rule of God, not as an internal experience, but as an external reality (Daniel 2:44).
Also as noted by many, it would be strange for Jesus to tell the Pharisees who were hostile to the true Kingdom of God that it was "in them."
However, "The kingdom of God is in your midst" as a reference to Jesus himself being in their midst is also a bit strange. I thus propose the following interpretation that emends the traditional punctuation.
It must be remembered that the original manuscripts had no punctuation. The following translation makes absolutely no change to the text itself, only to the punctuation. The change is subtle but extremely significant:
Being asked by the Pharisees when the kingdom of God would come, he
answered them, “The kingdom of God is not coming in ways that can be
observed, nor will they say, ‘Look, here it is!’ or ‘There! For look!
The kingdom of God is in your midst!'"
The crucial change is where the final quotation marks are placed. This punctuation makes it clear that Jesus was not telling the Pharisees that the Kingdom of God is in their midst (nor in them), but is saying that when the Kingdom comes, people will not be saying: "Look! The Kingdom of God is in your midst!" This punctuation puts this statement in the mouth of the false-messiahs (see Matt 24:23).
This rendering is supported by the parallel ιδου (Look!) which is often somewhat obscured by translating the same word two different ways. For example, the NET Bible translates the first ιδου as 'look', but the second ιδου as 'indeed'. Other translations have 'look' then 'behold'.
So my conclusion is that while "in your midst" is superior to "in you" in view of the nature of the Kingdom of God, Jesus did not mean the Kingdom of God was in the Pharisees, nor did he mean it was in their midst, but that when the Kingdom of God comes, people will not be saying: "Look! The Kingdom of God is in your midst", because it is not coming in ways that can be observed.