All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
According to the preponderant opinion of scholars, it is permissible to participate in these lots on condition that the person is not buying the item for the sole purpose of taking part in the lots without any need for that item, and that he buys it for its market price without any increase.
So, if this was the case and you intended to give your sister this coupon as a gift, then this car would be hers. Her guardian should dispose of this car in the most beneficial way for her by selling it, using it, and so on.
However, if you did not intend to do so and registered the coupon in her name out of optimism and as a good omen of winning, then, this car belongs to the owner of the money with which you bought the item.
Allaah Knows best.