Latin root sist means stand

Comes from Latin sistere, the second-person singular future passive indicative of sisto means stand.

resist

Stand can be aggressive when it's done actively, because it's a fighting stance, when it's done passively, it means hold, keep, remain.

Another root tain has the similar meaning, tain means to hold, the difference is you hold with your hands, but stand with your legs. For example the retain and resist, retain is to keep something to not to be affected, think about grab something back, there is no aggresiveness here, but when you resist, you are doing almost the same thing as retain, but in a more aggresive way, you stand for fighting back. Notice the two backs here have different direction, in retain, the back is this side, in resist, the back is the opposite side.

insist

Stand(sist) upon(in), means declare something strongly. What is the instinct action you will do when you want to defend your view point in an arguing or to assert the ownership of something like property in conflict? Of course it should be standing up, the stance is an aggressive signal, telling others you are preparing for fight to defend the things you are insisting.