I think mentioning gifts, whether to bring them or not, is kind of a no-no. I understand you don't want people to feel obligated, or for you to look gift grabby (totally honorable) but by bringing up gifts at all I think you kind of put it in people's head.
Some people will bring gifts no matter what you put on the invite, and some won't because a sip and see is not necessarily a gift giving event. Def. wouldn't register because that would up the "feeling obligated" factor, and if people ask what you need you can say, "honestly we don't need a thing, we just want to see everybody!"
You could also try to talk MIL down to just hosting a dinner/bbq that isn't really baby related, but she seems like a pushy broad, so good luck with that :)