Please help me remeber the title of this film!!

[CorinnaJ]

Both me and my brother remember a film from when we were kids, probably made in the early 1980's. It was about a boy and his sister. The boy was allergic to precious metals, if he got near them it hurt. He was kidnapped, as obviously useful to find treasure, and the sister spends years looking for him. I remember there was a bit about a plague and various people helping her. She finally finds him and he has learnt to control his 'skill', so that it no longer hurts if he is near precious metals. However, he has turned nasty and greedy. Not sure what happens then, but at the end the castle collapses, he turns nice again and he sketches a flying machine (glider) on a rock - bit da vinci like in design.