How to deal with a sister that isn't talking?

I've got a sister who is 1 year older than me. She has decided she isn't talking to me. It sort of became obvious that she wasn't responding to messages etc. So I sent her a message "Have you decided you aren't talking to me?". I know it was read and there was no reply. A couple of weeks ago she told my 15yo daughter that she wasn't talking to me. Funnily enough my daughter knows my aunt is being childish.

My mum lives in a granny flat behind my sisters. She is going to have her 80th soon and it will be at my sisters place. I'm thinking that if she isn't going to talk to me it would be very awkward and perhaps I won't go......