I need honest advice urgently?

my wife was accussed of kissing my mate years ago by her sister,at that time we had a kid and we where living with her parents ,we met this nice couple and my wife would go up to her house for drinks ect ect and became very close to her,about a year later my wife sister was telling people in her circle that my wife pionately kissed my mate outside a night club (at the very start of our friendship ) but at the time i confronted my wife and she denied it but her sister was adament she did,i could not pursue it at the time as i have no family and i had no where to go and was more or less told by her parents to let it go as it caused a mive rift with them,but now i have became a succesful bussinesman and stand on my own two feet ,i feel its time to get the two sisters together and get it sorted ,this was 15 years ago but i cant get it out my mind the fact that possibly my wife befriended his wife knowing that she kissed her man,this is so not like my wife but i feel now i deserve to find out,my wife and sister inlaw have never spoke to me together about this but it has caused a rift in the family for years ,should i stand up now and get answers as its pulled me down everyday for years ,although i know they will all make a fuss about this