Should we accept the rishta for my cousin sister despite her khala’s son’s past?

Assalamoalikum. My question is regarding my cousin sister. Her nani and Ammi talk about her rishta with her khala’s son in childhood. It was Ok for upto last 4 yrs. now the we heard that he, her khala’s son who work in abroad having an affair/relation with ‘gairmahram’ woman. We not seen with our eyes, people who stays their they talk about him. My one of chaha, her Mamu and other relatives also stays their but no one says confidently about his relation. But other people always talk about his charecter. My sister is mashaAllah beautiful, well educated, follower of deen, hijab, very respectful and littlebit stubborn. And we heard so about her khala’s son. My chachi(sister’s Ammi) passes away four year back. From last three years my sister told that she dont want to marry with him (her khala’s son). Upto this it was ok. Now she is interested to marry with him even she know the everything. All my family is apposing to this. When I asked my sister why u taking such dicision, that u r refussing from last three years(She didnt talk/kept relation with that boy(her khala’s son) from last three years)She told I want to be in touch with my father in future. Her khala will understand her and there will be good co-ordination between them. And told give him a chance to be a good man. And if I marry someone else what guarntee that the fellow will be alright he will understand me. For her so many rishtey came she all refused. Now she/we are in very confussing mode that what can we do. What step we should take. Please suggest shall we accept this rishta because of her thinking. Try to reply on my mail id(let it be personal) Jazakumullah. Allah Hafiz