DMum and DSis had a huge falling out at the beginning of December and are still not talking to each other.
I don?t quite know how it happened. DM had gone to babysit for DSis during the snow and turned up a bit harassed after skidding on the ice. DSis somehow took this as DM not wanting to babysit and exploded, calling DM selfish etc. DM retaliated by saying that she never wanted to speak to or see DSis ever again 
Now DM is in a right state. Dad called the other night to say she won?t get out of bed in the morning and I think she is actually clinically depressed. She is still saying she doesn?t want to see my sister and cannot understand how DSis could have said that she is selfish. Meanwhile, DSis is merrily carrying on as if nothing has happened and has emailed my mum to say that she?s ?pathetic? and should have gotten over it by now 
With Christmas coming up I now feel like its reaching crisis point. Is it fair for me to carry on ignoring the situation every time I speak to my sister, even though I know my mum is so cut up about it all?