Whats the justification to determine the phase sequence?
CIVIL: In a C I leads V which leads I in an L.
Assuming your phase sequence is V1, V2, V3,  then the current in C1 will lead V3 and end up being closer to the phase of V2. This will reduce the current drawn by phase 2 relative to phase 1 and therefore the voltage will reduce on R2. Obviously reversing the sequence will give the opposite result.
Apparently you can replace R1 and R2 with lamps while choosing C1 to have a similar impedance value as the lamps to get a simple and cheap indicator. I've never seen one in real life.