Is it safe to wash a little Sterno down a sink drain?
Sterno is gelled denatured alcohol.  It is denatured alcohol (a combination of methanol and ethanol) so that you can't, safely, drink it, but it is still mostly alcohol, which will dissolve in water and clean up just fine.

While I wouldn't dump a lot of it down the sink, you should have no problem with cleanup.  And while Sterno isn't safe to drink, it won't be a problem if you touch it in cleaning up.

I just double checked the Sterno site and they confirm water cleanup and say, additionally, that it is biodegradable.