My mum is cheating on my dad with my uncle

My mum is cheating on my dad with his brother online. I think they've kissed but I'm not sure. They live in different countries.

But when I went onto my mum's phone, I saw all these sexual and loving messages between them.

I've already told my mum's sister but we have no idea what to do. As far as I know it's been a year since I found out. Please help me. What should I do?