Be prepared for some potentially upsetting changes in your dm's personality. I'm not saying that will definitely happen, but it is one aspect Dh in particular and I were unprepared for and really struggled with.
There were days when she was very down and reflecting on her life in a very negative way - that was hard for Dh to take, as he has a wonderful childhood with her and had somewhat idealised his parent's marriage.
There were also days when she was hugely, hugely angry and would lash out at anyone and anything. Days like that you really couldn't do right for doing wrong.
Towards the end, she developed dementia like symptoms as a side effect of treatment and could be pretty nasty. Of course, this wasn't her, but the illness. Still very upsetting.
Dmil was a wonderful wonderful woman, but she was also a human being and knowing you are going to die must be utterly terrifying.
If possible, make sure your dm has someone outside the family that she can unburden to - macmillan shoukd be able to help or sign post with this.
And also make sure both you and Dh have someone outside the situation to talk to - sometimes you need to offload and rant etc. It's easier to do that with someone who isn't emotionally involved and struggling with their own feelings.