OP is getting a hard time.
She doesn't sound entitled, she didn't ask for the money, she doesn't know the Uncle. The sister promised 50/50 split in childhood. I think that's a promise that can be ignored.
She promised 20% as an adult with 3 children. The OP understood.
A week before the Uncle died she said again that she would look after the OP.
Now she says she was never serious in the first place.
OP isn't annoyed about no inheritance, or that she isn't getting her hands on any....the sister has repeatedly promised as an adult that she would share with the OP, and now she hasn't.
Of course she can change her mind, a promise means nothing, it's not a contract, but the upset is because clearly the OP's sister has gone back on a promise made several times.
400K is a life changing amount. To offer a meal out when you've just inherited 400K, after promising more, well....it's a bit shit.