We had a lot of people fail to RSVP if they weren't coming; we ended up deciding that if they didn't let us know they weren't coming, either through RSVP, phone, letting me/close family know, then we wouldn't count them. And none of them did show up. We also had a few people who did RSVP not show, mainly older relatives who weren't well enough to attend on the day, so even if a few extras had turned up we would have been okay.
From the other side of it, a lot of people don't know if they can get the time off work/if they can find a babysitter/if they have other plans at the time the invitations are sent out. And sometimes it just slips their mind. I know I've lost a wedding invitation before RSVPing, though we made sure to let the groom, who was a friend of my husband's, know that we were definitely going.
I know my husband's work won't let their employees book vacation time more than two months in advance. We've been screwed over a few times where we've had to book a vacation more than two months ahead, but by the time he was allowed to request the vacation time he couldn't get the time off.
So, basically (there is a point, I swear); it sucks when people don't RSVP, and yes, it's rude, but it's not the end of the world. If your mother is going to ride herd on the non-RSVPers at Thanksgiving, maybe have her make it clear (and make it clear to HER) that if you don't hear from them in some way before your RSVP deadline, you will be assuming they won't attend.