To be honest - I say that line but keep it to a minimum. I normally remind the person of the Mercy of Allah SWT and Insh'Allah her grave is widened and filled with Noor.
I just feel that saying you sorry over and over again just saddens the person more - I prefer to concentrate and lead them to be more positive about the deceased abode being a pleasant one with the mercy of Allah SWT.
its a very difficult time to loose someone in a family especially such a close relative as a mother
So you could maybe use this time to bring your family closer together if you wasn't before, make dua for them and the mother in her grave
And keep an eye on the family check in on them, as many people can fall into severe deppression after the death of a loved one (deppression is no joke & can lead onto further serious issues)
(You don't have to do all this just suggestions that can be helpfull but would defo just remind them they will be reunited with their loved ones if Allah permits it etc etc etc Then you talk more about Allah changing the subject to not sadden them more)
I dont say anything bcos she wont be in mood to talk or hear .ill just sit there beside her .to a close person i sit till she is ok to talk and then ill ask of her the details .inspite of tears poring they will be in a way releived to share their misery with family nd closefriends.
Console her by saying everyone has to go back to Allah. tell her that her mother 's gone and So we will, later one day and we will be reunited with her Mother again FOREVER in Jannah inshallah if we die in eemaan , so lets try to be as pious as possible.
Also console the loss of a Mother is big loss which Only time will make it heal iow with the passage of time we will come out of that pain , inshallah