time constant confusion of second order system
Start with naming the middle node Vx, then you can say that: -

\$V_{OUT} = V_X \dfrac{1}{1+sCR}\$

You then have to find Vx in terms of Vin - this is the start I'd consider making but, be aware, the algebra becomes more complex because in finding Vx the component to ground that Vx develops across is C in parallel with R and C in series.