How do you deal with a meddling mother-in-law?

I simply don't allow it to happen. Since day 1 of our marriage, we lived on our own, far from the in-laws. It helped us a lot, given the fact the we are still building foundation for our new family. Based on experience, it's better to avoid things or people whom you know you could have problems with. And it works for us. We solve our own problems and we're very much independent as a couple and a family.