Well, I sort of saw the sandbender as a mix between Earth and Air. Though I may be wrong, I believe they had used the sand to create whirlwinds to move their crafts. The air bending properties allow them to move the sand around freely, unlike earthbenders, and then the earth properties allow them to harden it... This also explains why Toph had trouble seeing in it AND why she was able to collect it and harden it to stand on while preventing the castle from sinking.

Toph said that the sand was all "fuzzy" and that was why she couldn't see, also commenting that there was nothing wrong with fuzzy in Appa's case, and also that it was too "loose" for her to see properly. She picked up some sand and let it slide through her hands. The sand benders use the fact that the sand can be moved grain by grain to ther advantage and then are able to whip it up to cause wind-like attacks which would only be available to an airbender.

How Toph was able to collect the sand is a much more confusing topic. You might ask "How can she do that if she is not a sand bender?" which would make perfect sense unless you see it as another form of earthbending. The fact that Toph was not able to attack using the sand shows the earthbender's reliability on the structure (so to speak) of the earth. It does not move freely and individualy grain by grain, but as one.

This could all just mean that sandbending is an adapted form of earthbending... but I see it as a combination (though not direct) of airbending and earthbending.