What's even worse than overlooked RSVPs is a lack of response on a gift. It drives me nuts, because I sometimes don't even know whether they received it or not. I once waited over a year to find out whether a cousin had received a wedding gift of an entire setting of her sterling silver flatware! (And then I suspect that her mother ended up writing the note.) I have stopped sending gifts to family members who never can be bothered to send the simplest "thank you" or phone, under the assumption that the gift couldn't have meant much to them.
I don't hold it against them, but save my energy.
When we were little, my mother would make us write "thank you" notes (or pictures) by the time we could write our names. In fact, the house rule was that you could not play with it, wear it, eat it, or spend it until
after the note was written. We did have a few obstinant temper tantrums that resulted in items returned, which elicited tantrum #2. I thought it was a cruel rule then, but feel very differently now. When the children are not properly socialized, people in helpful places believe them to suffer from a lack of breeding. Not good for baby's future!