my dad cheated on his new wife with my mom

Now quite a storytime again. It was like two years ago. My mom and dad didn't divorced yet at that time but they were living in different cities. My mom was constantly cheating on my dad. I explained why in my second blog. So does my dad. He had his recent wife as his gf that time. My dad came to our home to bring some grocceries. 

He stayed like for three days. After he went to the city we moved from again, my mom toold me that she slept with my dad. So he cheated on his gf -the woman who is his wife now- with my mom.