I was eaten cakes by my younger brother.

如要说 你和你弟弟一起吃蛋糕就应是: I ate cakes with my younger brother.
如要说 我的蛋糕被弟弟吃了 应是: My cakes was eaten by my younger brother.

应该是from

绝对不对。
你想要说的汉语意思是什么，我帮你翻译。