I'm so sorry you had to go through childhood without your mother. What the other ladies have posted here is good advice. It's always easier to tell your children hard things when they are young and can work out what they need to know in their own way. Trying to protect them from bad news never works out the way we want it too and when they're older, only makes them wonder what else they might not know.
You are a great mom and daughter to be so caring of your family.
Best wishes for the future.