I am sorry I joined this thread late. And I'm sorry you have to deal with your intrusive MIL...
Sometimes it's so hard to be nice and polite without being stepped all over, isn't it? My MOTHER kept trying to get me to wean my baby (we're at 8 months and still going strong), and SHE VISITED EVERY WEEKEND. It's her only grandchild and she's been wanting to play grandma for ages.
I found that the best way to stop the discussion is not to engage. She would bring up weaning, and I would say "Breastfeeding is fun". She would bring up the what-is-best-for-baby-is-formula-milk topic and I would say "breastfeeding is fun!!" (even though there are times when it's not fun at all
) She would bring up the irresponsible-mother-needy-baby topic, and I would say "Breastfeeding is FUN!!!". And since she "forgets" that she said it all already, she would bring the same topics up every week when she visited. For 6 months.
For the past two months, it's been all about stop-breastfeeding-and-feed-real-food topics. And guess what I say?
Believe me, nothing makes me more pleased (not e-hell approved, I know) than to see her so frustrated at not getting her point across. After all, if I have to suffer her lectures, why not have fun at it?