英语冠词使用的问题刚才做听力，看到这两个句子Often a cat or a dog can comfort us at

英语冠词使用的问题
刚才做听力，看到这两个句子Often a cat or a dog can comfort us at times when human words just don't help.
第二个：
A pet can be something different to each member of the family, another baby to a mother and father, a sister or brother to an only child,
问题：第一句a cat or a dog和第二句的 a mother and father, a sister or brother 为什么前者前后都有冠词而后者没有？省略？平行结构？都说不通啊，就是搞不懂为什么第二句不加冠词，求大神解答。谢谢！
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