两个都可以,但有不同语法结构和用法：
I was excited to hear from my sister in New York.
I was looking forward to hearing from my sister in New York.When she did call,I was so excited.to后面的动词不是只能加原形不能跟正在进行式吗？不是这样的，你没说错，to 后面如果是动词，要接动词原型（即动词不定式），但第二句的结构是完全另一回事，和第一句没有直接关系，不要两个一起看，词组 look forward to 可以接名词短语，hearing from my sister in New York 就是名词短语。I was looking forward to hearing from my sister in New York. 和I am looking forward to meeting you.是相同句型。我提出这个例子是为了解释词组 to hearing from you 是正确、可行的用法。我是加拿大人，英语是我第一语言。