Is a woman rewarded for taking care of her husband's children from a previous wife, especially when residing in a non-Muslim country?

These children are a trust, and preserving them, caring for them, and raising them upon Islam will bring you abundant reward and great good from Allah Almighty, and they will be a treasure for you. The reward is greater with Allah for those who uphold trusts and covenants, who will inherit Firdaus (Paradise). Showing kindness to these children, especially in a non-Muslim country, magnifies the reward.