Well, it might be somewhat alright if your Aunt knew about it and is ok with the arrangement, and your Uncle's mistress knows her place in the family hierarchy and properly defers to your Aunt's authority...but this is everybody but her knows. But...This is a time bomb on your Uncle's lap, his affair, his problem. Just prepare the comfort stuff for your Aunt when the shit-storm hits. And for your mother and yourself to get slapped for knowing and not telling her.
I don't know how family is viewed in China, nor what family vows are made there when a person is married. But, if you say that your aunt doesn't know, then, I can say that your uncle acted in a bad way as he didn't have the courage to admit his behaviour or change of feelings - he didn't mention what happened to his wife, that's not a nice thing to do as it's more than a lie (he, actually, 'consumed' the relationship, no?). In my country (a monogamous country), marriage ends when the people divorce. One case (reason) for divorce (withouth the consent of the other person) is when the married person lived apart (separately) for at least 2 years. For a relationship there's a need for 2 people. When those people agree with the begining of that relationship or when they aknowledge that relationship, then, they can, officially, can say that they are in a relationship. If only one of them believes that he/she's in a relationship, then he/she's wrong as you can't force someone to be in a relationship with you - I'm talking from a social and/or moral point of view. A relationship/break up can be unilateraly decided - it's enough for a person that's part of a relationship to decide to break up. It has something to do with the liberty of decision that any and every person should have. Is your uncle the brother of your mother and the son of your grandmother? Your cousin (the son of your uncle) may find himself in a difficult position as he'll have to 'choose' what to do and what to say if he's loving both his parents. Beside that, there's also a moral dilema as he'll have to choose what to do, what to say - to decide if he'll 'support' that kind of lie and that kind of behaviour. Emotions/feelings change. What I disapprove to your uncle is the fact that he didn't have courage and he didn't 'solve' the old relationship. That's not correct, not to his wife, not to his son.. to for his family or to his new partner/love (?) - that'll create lot of difficulties. In my country, if adultery is proved, the one at fault can be held responsible for some (kinds of) damages. I'm not absurd to say that emotions can't change, but, before going and 'playing' with someone else beside your husband/wife/partner.. at least, have courage and be true to the promises and the feelings that you've admited to have. After you've solved out the old relationship, you can begin the new one. If you don't do that, then.. that tells something about the value of your words, about how you are, as a person and about how you treat yourself and others. In a relationship, some decisions are to be taken by those who are involved in it. I don't know how you should act, but.. you can try to think and 'put in balance' how you are, what values you have and how you treat those close to you. In the end, I think that your aunt will find out about what her husband does - that kind of things, in most cases, are discovered pretty fast and the consequences aren't always nice. Though.. I think that you should try to understand the situation better, to see the perpective of those who are involved: your uncles point of view, the point of view of his mistress(?), the point of view of your cousin and so on. Why? Because, maybe.. there are things that you don't know. Maybe, in fact, your aunt is aware of that but, for various reason (i.e. 'face' or social position) things weren't officialy clarified. Also, maybe, things aren't what it seems to be. There are too may things to be taken into consideration when thinking about how to proceed and I lack such details. I can only wish you good luck so, good luck! Here are some links that may be of help: https://www.quora.com/topic/Coping-with-Infidelity-and-Adultery https://www.quora.com/topic/Infidelity-and-Adultery
Can your uncle and your family can hide the truth from your aunt for forever? Imagine if someone else tells the truth to your aunt and how broken will be she. His man is cheating on her openly while having no shame and the family she accepted after marriage has betrayed her... If i was your grandmother or grandfather I would have beaten your uncle fucking hard and will tell him to apologise her wife instead of supporting him just because he is mine son. But I too will be in hard position if i was you. If you are really close to your uncle's family then think about it a little more and have a discussion with your mom or maybe your cousin otherwise pretend as a cat and forget all. Well it would be more shocking if your uncle and aunt child is less than 15. You know in China you will be fired from job if it is found out you have committed adultery. In india those who committed adultery are seen as piece of trash. It is very rare to find someone in india who have a mistress even in Muslim community who have 200 million population here, even polygamy is very rare in Indian Muslim even though there religion allow it. I have lived in China for 1 year due to my father's job and I don't think adultery is common in China except for rich guys. Whatever country is it rich guys will always be unfillial. Bill Clinton and tiger Woods are theperfect example of that.