If the first spell of my opponent got defied by me, can I still use crumble sand to counter the second spell they played?

No. You cannot use Crumbling Sands to counter the second spell because the first spell was Defied, and a countered spell is not considered "played" — so the condition "if an opponent has played another spell this turn" is not met.

Here's the full sequence:

1. Opponent plays Spell A → you play Defy targeting it.
2. Defy resolves first, countering Spell A. Per Rule 425.1.b, a countered card is not considered to have been played.
3. Opponent plays Spell B.
4. You attempt to play Crumbling Sands targeting Spell B. Its effect reads: "Counter a spell if an opponent has played another spell this turn." The only spell the opponent has played this turn is Spell B itself (the target). Since Spell A was countered and isn't treated as played, there is no "another spell" to satisfy the condition.
5. The condition fails — Crumbling Sands cannot successfully counter Spell B.

This is consistent with how "played" works for non-triggered conditional checks (e.g., Battering Ram also does not count a countered spell — per Rule 419.4.b example). The Legion keyword is a special case because its rules text (Rule 812.1.c) explicitly checks Finalization, not "played" — that distinction doesn't carry over to Crumbling Sands.