External interrupt doesn't set flag bit
This is probably because there are other functions on pin B1 that need to be disabled before you can use it as a digital input.

In the datasheet we find table 11-4, shown below:



A quick look at that register in the datasheet shows that it is 1 by default, which means analogue input mode:



You will need to clear this bit before you can use it as a digital input.

You will also need to reset INTCON3bits.INT1IF to 0 in your interrupt service routine or the handler will keep firing and you will never escape it.

If you are trying to measure PWM you might like to read section 20.2 "Capture Mode" in the datasheet.  This uses the Enhanced Capture and Compare Peripheral.  It acts like your edge selection interrupt but will also store the value of TMR1 or TMR3, allowing you to make measurements of the period or duty cycle (or both).