Seeking repentance for one's past sins before marriage

Q: If a man and woman commit zina before getting married. After their marriage, they had children and realized that what they have done before marriage was a sin. But they do not repent when being married because they dont have knowlegde that before marriage they had to repent.

After 6 years they got to know that they should have repented before nikaah, otherwise their nikaah is invalid. Is their nikaah valid or not as they repented after 6 years of marriage.

Taking a vow not to use Facebook and thereafter mistakenly opening someone else's Facebook

Q: One day my wife made a vow to Allah that she will never use Facebook in the future. Her sister uses facebook and always has automatic login into facebook on her mobile. When my  wife uses her sisters mobile, mistakenly she opened the facebook app and automatic logged into her sisters facebook account or profile. She saw her sisters Facebook profile picture etc. but she didn't use Facebook, she just saw it.

Meaning of transparent clothing

Q: Can you please tell me how do we determine if a dress is transparent or not. Most of my dresses are such that if I hold them in front of my eyes, I can see whatever is on the other side. But if I put my hand below the dress or wear the dress, my body colour cannot be seen. Are these dresses considered to be transparent? What is the way of determining if a dress is transparent or not?