Sincere condolences at the loss of your Mum
In addition to your loss you write about a very sad situation, a sad situation for you and for all those who are grieving the loss of you Mum.
As has already been said it "seems as if" your DD is having a tantrum.
From what you have said she is having an emotional outburst and you are A recipient of hurtful and unwelcome behaviour.
You may not be the only person she is treated this way.
Although when you are the one on the receiving end of this behaviour It Is Only Happening to You at that time.
Her behaviour hurts you, feels unjust and you expect more care and consideration from your Child than she is giving you at this time.
Not unreasonable to expect care and consideration and at the same time your expectations may be unreasonable.
You are both hurting.
Other have said grief takes many forms and the emotional hit from the articulated lorry of pain and loss that knocked you and your family sideways takes time to manage. So allow Time.
Time for both of you (all of you) to reestablish some sort of balance with-in yourselves.
Do not put a time limit on this process, grief has a time frame all of it's own and is different for us all.
Firstly finding any kind of balance/normality at this time is exceedingly difficult, work at accepting these real difficulties and then find a way to be Kind.
Be Kind to yourself.
Then find a way to show Love, Compassion and Strength to your family.
You are very probably feeling empty and lost right now and don't think you can find anything to Give - you are a Mum - you can do this.
Ask yourself what other stressors and anxiety is your DD dealing with in addition to the loss of your Mum/her grandmother ?
What other stressors and anxiety are you, your brother and other family members dealing with that also take make this painful time harder to deal with ?
Getting an understanding on "the other stuff" often helps towards a calmer understanding.
It would be lovely if you got that consideration too but be very careful about expecting the understanding of others,
You said you "needed your children now" it's a hard thing to say but your children need their Parent now - that's you.
Sadly in your own pain and loss you are still Their (Her) Parent.
Even though you are The Child who has lost her Parent.
The support you need (it is OK for you to have support) you need to find from friends and or a counsellor. Some one has already suggested Cruse, good suggestion.
Find someone who is not grieving for you Mum to provide support to you.
As for you DD behaviour ? It isn't a time to demand changes from her nor is it a time for you to take this behaviour from her.
Finding the balance between not expecting caring behaviour and not accepting destructive is never easy so I wish you all the luck you need in finding that balance.
If you want a relationship with her after the worst of the pain and grief has passed STOP asking, expecting, getting others to ask her for an Apology.
TIME.
This is not the TIME.
May be in the future but not now.
I hope you find a way to get some support for yourself and that your DD finds a way to get some support too.
Grief is hard on everyone and in families we often hit the different phases of grief at different times and that is an additional burden on relationships.
I hope you find a way through this pain and that you and your DD have a relationship without anger and rebellion at some point in the future.
Wishing you the best possible outcome in this very sad situation.