YOU SAID:
My brother has a cat who has a cat who has a cat who has a dog.
INTO JAPANESE
私の兄は犬を飼っている猫を飼っている猫を飼っています。
BACK INTO ENGLISH
My brother has a cat that has a dog and a cat that has a dog.
INTO JAPANESE
私の兄は犬を飼っている猫と犬を飼っている猫を飼っています。
BACK INTO ENGLISH
My brother has a cat that has a dog and a cat that has a dog.
You've done this before, haven't you.