I would place this book around pre-2010. A young woman is hired to babysit a boy (I believe he’s quiet, parents are dead or they abandoned him) and they form a special bond. I think they live in an old castle and it takes place in the winter. The boy’s family (I think there’s an old woman, the family is suspicious) dislikes her and go as far as lacing a (cherry?) pie with poison however, the woman doesn’t eat the pie. She also falls in love with a young man and they are pursued by an assailant with a gun (possible shootout) and they hide (behind stone pillars?) towards the end but they escape. I believe there’s a huge frozen pond surrounding the castle and there’s a great fear of going on the ice (most likely from the boy) and someone falls through. It’s a mystery/thriller.