Is a surprise prize from a usurious bank considered unlawful (haram)? And is it permissible to give it to a indebted brother to get rid of it?

If the prize is from a bank whose funds are mixed, and your dealings with it are in accordance with Sharia rulings, then the preponderant view is that it is permissible to benefit from it, though it is disliked (makrooh). However, giving it to those in need is preferable to avoid disagreement. It is permissible to give it to your brother to pay off his debt if he cannot find what to settle his debt with. If, however, your dealings with it are usurious (riba-based), then those transactions must be stopped immediately, and you are not permitted to benefit from the prize. Rather, you should take it and spend it on charitable causes and public welfare, or give it to the poor and needy.