The doctors told my parents that my aunt was suffering from cancer so they had to stay with her.

suffering 是动词 suffer 的现在分词形式，suffer 的意思是"遭受、忍受（痛苦或疾病）"。 from 是介词，表示"来自、由于"，在这里引出疾病的原因或来源。 suffering from 是一个固定搭配，表示"患...病"或"遭受...痛苦"，后面接疾病或痛苦的原因。