Reducing circuit to resistor and capacitor in parallel
Let me redraw the circuit, and hopefully it will become clear:







simulate this circuit &ndash; Schematic created using CircuitLab

Because we are interested in the impedance seen by V1, we want to reduce this entire circuit to a single impedance between V1 and ground (we don't care about V2). So:


combine C2 and C3
reduce C1/R1 to a single impedance (Z1)
reduce R2/(C2+C3) to a single impedance (Z2)
sum Z1 and Z2 to obtain the total impedance
separate the total impedance into real (resistance) and imaginary (capacitance) to obtain the "resistor and capacitor in parallel" part.


Performing these calculations in the \$s\$-domain will make it much more straightforward.