Thanks so much to you all for your advice. I think a support group does sound good for her and I don't think she has tried that - if it comes to the point where she is ready to take advice I will certainly mention it to her. She is a bit defensive at the moment. Yes, as you say it may take 5 years for her to get over.

I will certainly let my mother know that she doesn't have to listen to her venting on the phone or bringing up issues at family gatherings. Mum has been listening because she wanted to help my step-sister but it's not helping. We are a bit lax on setting personal boundaries in our family which is something I'm working on myself.