Thanks for your replies

I dont have any issues with babies that suck their thumbs, or babies that use a dummy, I just wont be 'taking his wee thumb and putting it in his mouth' like my MIL said I should do as that is 'what she will be doing'. I found it a bit annoying that she thinks she will be able to do this without even asking how we felt about it.
You're right though, I will just have to tell her straight.
I spoke to hubby about it today when he got home, he said not to worry and that he is going to say to her that she will not be putting our babies thumb in its mouth because we dont want her to! She will take the message from him for sure, I just didnt want hubby to think I was being over sensitive on the subject. Luckily he feels the same as me though, phew.
Thanks again