I remember an animated cartoon in the 1980's or 1990's about two children, a brother and a sister, who are in a world with a lot of snow, governed by a bad queen who lives in an ice-castle.
In the beginning, the queen is kind, but she want to corrupt the boy (who later became a bad guy with a chivalry-sword). The queen has injected ice in his heart.
His sister want to save him, with a boring side-kick character ... there was a deer with her, I think.
This was not very well drawn ... I watched it when I was a child, that's all that I can remember about it.