They complain about too much with each other.他們彼此抱怨太多。
complain to sb 向…抱怨
complain about/of sth 抱怨；申訴；自訴有...病痛；投訴
（例子來自牛津詞典）
The defendant complained of intimidation during the investigation. 被告申訴在調查期間受到了恐嚇。
I'm going to complain to the manager about this. 我要就這件事向經理投訴。