First, may I send you a hug, OP. This must feel very raw, and you have barely had time to absorb the shock before having this inquest added to the burden.
As an outsider, I can only imagine the mixed emotions and distress you are feeling, but I wonder if it might help to try and look at this from different angles.
Firstly, from your mother’s point of view - this was her own choice, she was extremely elderly, and presumably was tired and had had enough of life. I am sure she didn’t want to hurt you or your DDs, but she had to do what was right for her. Try to understand and accept her choice, painful though it is.
Secondly, your DDs. You have tried to be kind in glossing over the death as natural causes, and you have done this from the best of motives, to protect their mental health. However, if it comes out at the inquest that it was in fact suicide, your DDs may feel hurt that you lied to them. At that point, I think you would need to be completely honest with them, and explain that you tried to shield them out of love and concern, and that you are scared of losing them too. This might actually allow a clearing of the air, and let you all grow closer as you support each other - not a bad outcome.
Last, but definitely not least, there is you. You are grieving your mother, trying to shelter your DDs, facing a load of funeral arrangements, an inquest, and trying to juggle everyone else’s needs. Who is supporting you? You need someone to be your shoulder to cry on. Do you have a good DH or close friend, who you can vent to?
Your parish minister or vicar is trained in bereavement counselling, and could be a great comfort to you. You do not need to be a regular church attender, they will help anyone resident in their parish.
Prioritise your own needs, OP. Rather like “fit your own oxygen mask before helping others”, as they say on plane safety briefings. Be gentle with yourself, and just let yourself have as much time as it takes to come to terms with all this. You will be emotionally unstable, tired and fraught, and you must make allowance for yourself. Do the absolute minimum of chores or work, and accept any help that is offered.
You and your DDs will come through this, but it will take longer than you might think, and grieving is hard emotional work.
May I end on a positive note. You obviously loved your mother, or you would not be so upset at losing her. Nobody can take those memories away from you. There is a memorial carved in stone beside the Water of Leith in Edinburgh, that reads “Grief is not forever. But love is.”
Hold that thought, OP. God bless.