I am so sorry to read your story, and sorry for the loss of your much loved husband. Your deeply unpleasant sisters are monstrous. Legally they cannot stop you visiting your parents, nor from attending any funeral anywhere. As others here have said, it would be more than wise to inform the police of your sister's letter, so as to have it on record should there be any further mental harassment or physical violence against you.
Your wonderful marriage is your treasure, and your sisters' nastiness is perhaps pure jealousy of a relationship they have never achieved? They cannot take your marriage away from you, ever!
And I'm pretty sure the Court of Protection does need to be informed of the situation, as your father may have been put under significant pressure to change Attorney. Perhaps check this out on line, or in a free half hour session with a solicitor used to dealing with this type of thing.
Crying in your situation is normal! In any other country and in most 'normal' families you would be thought heartless if you didn't cry!
There's lots of great advice from others here about groups to help you with the grieving. The best advice I was ever given when my dearest relative died, was to make a point of eating regular meals and getting out into the fresh air and walking. Sounds simple - but it did help . In a way you have already taken the big first step to self help by sharing your problems here.
Take heart for next step, whether it's counselling, joining the WI, or U3A, whatever you choose, you can do it, and life will slowly, very, slowly feel better. Look after yourselfabove all, (and the legal stuff, it does matter!) I wish you courage, luck and to be happy again.