Sorry Hypnorm, just saw this. All research done on this has actually showed the opposite of what your mil is saying. As babies have better co-ordination in their hands rather than their mouths, they are able to talk back to you at a much earlier age. By learning to communicate with their hands they are able to apply what they have learned about communication to their voices when their mouths have the muscle strength and skill. As someone else said, you should encourage their spoken vocabulary by speaking as you sign.
Everyone I know who has signed with their babies have had children who cry less, are very polite (they sign please and thank you very early!!), and have a much higher vocabulary when they get to talking age. Funny enough, most of the students at my school with younger brothers and sisters who have grown up signing also have much higher reading abilities than their peers.
