What should we do about sil?

Sil not letting us see Kids

My first post here. My mother and sister-in-law fell out for the first time in the summer because my brother (sil's husband) was in hospital for six months and she didn't visit him. My brother has mental health problems and sil was never supportive as she claims he is fine! Despite being section three times during their marriage. 

We were very close to the two older kids and are missing out on the 1yr old growing up. My mum and I have been helping sil for years with looking after the kids and financially but she's ungrateful cow saying we've not done anything for her. 

She's turned the 11 and 13yr old nephew and niece against us and my brother is still in hospital. We miss the kids but we can't forgive her for how she's treated my brother and especially for never visiting him in hospital. He once stabbed himself in a suicide attempt and she refused to call an ambulance as she ''didnt want the neighbours to find out'' we forgave her for this and many other dangerous she's put my brother and nieces/nephews in. 

There are rumours now that she remarried (although still married to my brother) someone whilst on holiday in Sharm el sheikh! Leaving the kids including toddler with a relative dying of cancer.

She's very selfish and we don't really want anything to do with her till she changes. Do we have legal rights to see the children? How can we have an amicable relationship with this woman for the sake of the children? This is breaking our hearts as we we were very close to the kids.

My brother was a great husband to her in their 14 year marriage but he would have relapses now and again and go into hospital. But 90% of the time would do all housework, shopping and helping kids with homework, taking them to school whilst she recovered in bed from nights out with her friends. We understand that he can't be easy to be married to someone with mental health problems but what kind of human being does not visit their ill husband and father of their children in hospital?