I had a question regarding this card ( Dorea Sand ) and quick forum search did not provide me with an answer.

Dorea Sand's Response:

Response: After you lose a challenge as the defender, until the end of the phase Sand Snake characters you control gain: "This character does not kneel to attack."

So following situation:

I have Red Viper (TftRK) in play and also Dorea Sand. I lose a challenge and trigger the response of Dorea Sand, so all Sand Snakes do not kneel to attack until the end of the phase. After this at some point in the same challenge phase, the Red Viper dies to any card effect. I play No Use for Grief and bring out all Sand Snakes remaining in the Deck. The question is now: Do all the newly in play Sand Snakes also dont have to kneel to attack?

Thanks in advance!

Yes. Dorea's ability is a general lasting effect affecting Sand Snake characters "you control." The identity and number of those Sand Snakes is not set in stone when her effect is triggered.

Perfect, thank you.