Confused about filter's cutoff frequency in rational sampling rate converter
I assume you'r talking about digital interpolators.

An interpolator is basically an L times expander followed by an anti-imaging lowpass filter with gain L. The cutoff frquency of the lowpass filter is defined in the discrete-time frequency domain as 

$$ w_c = \frac{\pi}{L}. $$

The analog equivalent of this cutoff frequency is

$$ f_c = F'_s \frac{ w_c }{ 2\pi} = L \times F_s \frac{ \pi / L}{ 2\pi} = F_s / 2. $$

This frequency refers to the increased sampling rate $F'_s = L \times F_s$.

A similar argument holds of the rational sample rate conversion system where an expander (by L) is followed by a lowpass filter and then followed by a decimator (by M). 

The cutoff frequency of the intermediate lowpass filter is $$\omega_c = \min(\pi/L, \pi/M).$$ Then the analog equivalent of this lowpass filter will be
$$ f_c = L \times F_s \frac{ \min(\pi/L, \pi/M)}{ 2\pi} .$$