I’m not sure what happened but when it comes to me and my brain, I forget..... often and quickly. I read a text and I don’t reply for weeks because I got busy and then forgot. I wasn’t purposely ignoring someone. I’ve often been told it’s my nephews birthday today- I forgot to write a wish. It wasn’t intentional and it wasn’t malicious.
Have you spoken to them at all since you had that fight with your sister? Perhaps they might be thinking you’re mad at them as well because of their mother.
I find assuming causes the most problems. We assume someone is mad at us but when they’re not. They could be easily suffering from depression and avoid the people that make them feel, people that will see through the glamour.
Either way, I hope you find answers and peace in this situation