Sometimes turning it around so that you throw the question back at her can help - as in 'so what advice did YOUR mum and MIL give you - and what had changed between the time they had children and your children?'
Sometimes they remember that they 'obviously' followed the current best practice guidelines and things had changed from when they were babies. Then they realise that things might have continued to change once they had stopped having babies. Pointing out the gradual change in advice that has brought down the numbers of cot deaths significantly is always a good one to stick in here, as is the fact that things are changing all the time, even over the last two or three years and there is different advice (have found with a 3yr gap between ds1 and ds2).
If they say they did exactly what their mum/mil said to do, you can then try exploring the 'don't you think that things have changed since grandparents had children...?!?' approach.
It is also worth asking them if they followed best practice at the time (unlikely they will say that they didn't!), where they found out about it from and why they don't want you to do exactly the same thing... it also manages to change the focus slightly so that, although the details of what you are doing might be a bit (a lot ) different, you are using the same approach they did - ie finding out current best practice...
And if not, don't let it wind you up, turn it into a game and see how quickly you can get her to say the phrase/how many times/about what/shout 'bingo' everytime she says it/etc. And promise yourself rewards depending on how/when etc and how right you are about your guesses are as to what her responses are. So if you thought she was going to say it within the first hour of her seeing you and she does, you get a bottle of wine/box of chocs/nice book/whatever reward you fancy. But if you think it is going to be the first hour and it is the second hour then you get a glass of wine/bar of choc/magazine/smaller reward... hope that makes sense!
good luck!