I think my mother is cheating on my dad

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I stumbled on something that has me confused and worried. I saw my mum kissing a man who dropped her home late at night. This shocked me and I decided to investigate to see if she is having an affair. I have been checking on her phone because I know all her passwords and I think she is actually having a relationship with him. They communicate frequently and although I am not sure, I interpreted one of those messages to mean that she is pregnant and she was asking him what to do about it. My dad is around and even though they sometimes have problems, he loves us very much and works hard to provide for us. Now I don't know what to do about this and need your advice on what to do here. I am 19 and the first born in our family. Please advise me.  {Lawrence*}