Here are the rewritten sentences containing relative clauses:

1. My sister, who lives in Australia, has three children. (MĐQH không xác định - cần dấu phẩy)

2. The waiter who was wearing a blue shirt was rude. (MĐQH xác định - không cần dấu phẩy)

3. The money that belongs to John is in the kitchen. (MĐQH xác định - không cần dấu phẩy)

4. The customer liked that waitress, who was very friendly. (MĐQH không xác định - cần dấu phẩy)

5. We broke the computer that belonged to my father. (MĐQH xác định - không cần dấu phẩy)

6. I dropped Paul’s glass, which was new. (MĐQH không xác định - cần dấu phẩy)