Shao Yong and Cheng Cheng are both from other places, and later both lived in Luoyang, so they are both from Luoyang, and they also met and knew each other in Luoyang. Shao Yong and Cheng are teachers and friends, but aren't they cousins?
As the saying goes, "a watch is three thousand Li", on the one hand, cousins can be distant relatives, on the other hand, they can also be said to be distant relatives. In Jiangxi and Hunan, everyone calls each other cousins when they meet. From this perspective, Cheng and Shao Yong may also be cousins, but it depends on the level of expression and the level of relatives.
If three people are close relatives, the poems and letters exchanged between them should be reflected in the title. So far, there seems to be no such relationship. The chronology of the three people does not show this.
It can be seen that the three people have a good relationship, respect each other and have more contacts than their cousins. Maybe there are cousins, far away, even far away, so close that the three of them have no time to mention them or can ignore them, so there is no need to delve into them.
There is even a folk legend that these three people are closer than their cousins, but it is not credible. Sometimes in order to attract people and enrich themselves, there are occasional folk sayings, which are not bad. Nan huaijin's teacher is very dirty, mainly because of this.
If people want to believe it and get it, it is best to have historical records and textual research, or extensive cognitive reasoning.
A little personal opinion and different opinions, please forgive Mr. Zhang.