Mum is having a sexual relationship with stepson

Hi I need some advise on my family situation that is somewhat confusing it all began 26 years ago when my mum left my dad (had 2 children together) and began dating his brother (my uncle) who had 3 children together. just recently my mum and my uncle split up (they never got married) and now she is saying she has strong sexual feelings for her stepson from my uncles previous marriage. I would like to know peoples views on this as we are all in a very awkward situation that usually happens in soapland!!

I also forgot to add that my uncles son from a previous marriage has been living with his uncle and stepmum for the last 15 years and we all class him as our brother!!