Thank you to all who offered constructive advice (MrsThierryHenry ) and kept an open mind, especially toughdaddy. That would have been the exact approach I would have taken if I'd have spoken to her.
However, I have made up my own mind to keep quiet for the time being.
In reply to some of the comments, especially the 'what goes around comes around comment' inferring that my brother was an arse or a twat for leaving his wife.
You don't know my brother so please do not make sweeping statements like that. He nearly died of cancer at the age of 29, at which point I think most people re-evaluate their lives. He hadn't been happy for a long long time and decided that life was for living. His 1st wife never shed one tear about his illness and yet cried for days over a bloody cat being ill !! My parents would drive 4 hrs to clean the house, look after the baby, buy the food, pay their bills etc. and she couldn't be arsed to even make them a cup of tea or say thank-you !!!!
She didn't make him choose her or ME, she couldn't bear him being close to anyone but her. We were all very sad it ended and my parents still made every effort to support her at the time.
His now wife is an attention seeking flirt, no two ways about it. I really like her but that's what she is. She'll even stand in front of my 68 year old father with a crop top on swinging her hips an looking down at her figure ... poor dad just doesn't know where to look.
Bad sex life ? My sil moans that she can't be bothered with it so why go flirting elsewhere ?
My brother didn't want to get married again but he did to please her. He certainly didn't want another child which he made blatantly clear at the start of their relationship - she got her own way there too. Everything she has ever wanted she has basically got. If anything my brother has been too soft with her. She comes home from work and would rather read the paper than give her child the attention he needs.
I know he is my brother and yes he has his faults, as do all men eh? but I just don't understand why she's doing this. I've seen her flirt with the ugliest of ugliest men, old men, young boys and married men.
LadyGP - My brother had an anonymous phone call from a woman saying 'I think you should ask your wife what she is doing with ...'
She'd been away on a course and had actually met this latest email/ txt chap. The emails were pretty explicit in what they were going to do to each other. That's what led my brother to the evidence.
She wants another baby but my brother had to have the snip because of further testicular problems (he had testicular cancer that went horribly wrong). Through all his recent worries of whether the cancer was returning all she could worry about was her and whether or not my bother would be able to father another child - their child was a miracle as his sperm count was basically non existent.
I hope that make things a bit clearer for some who seem to think my brother is an arse - which he is not. He is a hard working husband and father who just happened to fall out of love with his first wife, meet and fall in love with another woman whom he has given everything to. Is that a reason to think he is a terrible person ?
I am a sister who cares deeply about her younger brother and whom is worried about his unhappiness at the moment. of course I want to help, I came on here asking for sensible and sympathetic advice. I'll think more carefully in the future ...