首先，按照你的例子，Her house is opposite to mine。这里opposite 是形容词！
但是，你第一句话是有问题的！！！查牛津词典，opposite作为形容词：
1，[only before noun]?on the other side of a particular area from somebody/something and usually facing them
2，(used after the noun)facing the speaker or somebody/something that has been mentioned
3，[usually before noun]?as different as possible from something
词典里写得清清楚楚，opposite 作为形容词只能修饰名词，不能跟在be动词之后！！
第二句话对应上面第二词条，所以也是形容词