Hi L.!
I am with everyone else on this one... It only takes a moment to RSVP. And like one person said, adding an email address makes it that much easier, but seriously picking up the phone and dialing a number has been done for decades,so there are no excuses.
For the people who didn't show up and RSVP'd Yes, that in my opinion is one of the biggest problems you mentioned. I would for sure call them out on it, you paid good money and they didn't show, so make them squirm a little.
As for the people who didn't RSVP at all, well I would think twice about inviting them again, family, friend, I don't care... And if they ever had the guts to ask me why, I'd say I've invited you to things before and you never RSVP'd, so I figured you were no longer interested in attending my get togethers. Seriously, what could they say....
There is nothing wrong with standing up for yourself and not being a doormat, even with family and friends. I am only 38 and I've learned over the years family can be the worst when it comes to this type of thing and if you don't correct it early on, they'll continue to steam roll you.
I don't think anyone is obligated to send gifts or cards when not attending an event. If they do that's wonderful, but never should be expected.
Sorry this all came about for you, but a friend once told me, people or things that drain you of your energy should be removed from your life (and yes that includes family and friends). You only get to live this life once, don't waste it on people who aren't worthy of you or your family. Concentrate on those people in your life that truly care about you and yours and you'll feel much better!
Good Luck,
J. in Macomb