That is so sad. What a terrible thing to happen to your sister. It would break my heart if that had happened to our daughter.
RSVP means Yes OR No. You need to let the Host or Hostess know Every time.
They took the time to invite you or your child (which is considered an honor), surely you can take time to email, text or call? .
No excuses.
Let me give you an example of why this could be very important. Our daughter was offered the opportunity to interview for the highest Scholarship available at one of the Colleges she applied to.
As with all of her acceptance letters, she wrote a note.
She thanked the colleges and then if they needed to interview her, she also responded, yes or no. She did this by Note and email.
At this one particular College, after the in person interview, she was told this scholarship had been awarded to her if she was interested.
She thanked them and told them she would need some time to consider it. We went back to the hotel, she immediately sat down and thanked them in a hand written note, for their time and again said she would be in contact within a week.
She did not accept the offer. She called them as soon as her decision had been made. This Person told our daughter, she had the best manners he had ever experienced. This is a College in Georgia, we considered this compliment high praise indeed.
I did not prod her, I did not hint to her, I did not tell her to do these things, This is exactly how she had always been taught to respond to people. Even as a toddler I had her scribble her name on a note thanking people.
She knew if she was invited to an event, she needed to let these people know one way or the other. If she felt shy about it, she could write a note, if she was not shy, she was allowed to call them and tell them her reply.
This can be taught, but you do need to talk about it and model this behavior.
I can not say it enough, manners could make the difference between you and a crowd of other people. Same fr your children.