If you haven't already done it, try to make the crawlspace as air tight as possible. It could be just a couple places where the wind is whistling in and freezing a short section of the pipes. Make sure all ventilation louvers for the crawlspace are closed. You could even wedge a piece of 1/2" foam board on the inside (or outside if you don't care how it looks and can't get to the inside) of these louvers if you have them to cut down on air infiltration.

Some other random ideas off the top of my head...

Is there a way to safely place a small ceramic or oil filled radiator heater in front of the sink on the coldest nights?

Is there access to the crawlspace from the living space where you could place a small fan to circulate heated air under there for a while?

Others will be along and have some better ideas I'm sure.