Condolances to you, Op. What a shock!
You will pull through; all the best.
The little girl needs you to be the main decision maker and her main protector. Great that you are already close.
Always accept help, before you think you desperately need it. You seem like you have a caring extended family.
It would be worth charting a yearly plan for holiday care etc.So nobody is surprised or put out to have your niece for a holiday.
Start up a Go-Fund-Me page for educational fees and extra curriculum activities, and housing for your niece. A target of 500000 could be your aim. You could be responsible for the fund until she is 25, at which time she could access the funds for housing and education herself.
My neighbour's sister had five children (in the 1970s) and that sister's DH's brother and wife also had five. The BIL and SIL died leaving five school aged children.
My neighbour's sister and her husband took all five in, bought a minivan and became a remarkable family with ten children.
They were truly inspirational and all the kids did well. Kept the family home rented out for the kids until it was sold for the five to have cash to fund their own house purchases.
My school friend grew up from a new born with his Uncle and Aunt and cousins because his father, who was left with three other children and a newborn when his wife died was unable to care for the baby.
A fellow I was in university with was one of eight children when his parents died in a car crash. A foster carer and her husband cared for them all like they were their own kids. They were amazingly kind and generous. My friend was quite sad because of loosing his Mum and Dad when he was sixteen though.
Another local woman has taken in her two nephews due to their parents addiction problems. She is a nurse and single. She sold her house and bought a very large one and told the boys that it would always be their home - sweet hard working woman. The boys are behaving well and are successful at school.
There are a few instances in our family tree, back in the early 1900s, where newborns were raised by their Aunts or Uncles - slotted into their family - when their own parents died in childbirth, or of TB, or were unmarried without support.