I missed asking a cousin to my party last year. I invited her siblings and can't think how she wasn't on my list. She noticed, before the event, and it got back to me. I phoned to invite her and at first she said "sorry but they were busy that day" and then luckily, she relented and came along. I was mortified by my stupidity. We are still friends
Maybe you could invite the neighbour to something?
we really do need a middle of the night moderator