The book was the adventures of a brother and sister, who lived in a castle on an island in a lake. Their parents were on a long journey and their uncle looked after them.
They had to fight off a lot of baddies called Baugrens; if you could kill one it turned to bones in a split second.
I remember the heroine, who must have been about 12, wore a dress which was supposed to touch the floor. As soon as her parents left the castle she "cut a foot off the hem."
Anyway my mum thought it too scary and binned it.
I haven't seen a copy for 40 years.