I have the same problem with my mother, mostly. It gets so bad that she will just take him to get his hair cut without telling me, or basically TELL me that he needs this or that, and I should do soething different. I finally got ot the point where I got fed up with it. It seemed like my son thought she was his mommy, instead of me. I finally just told her to back off. I let her know that, if she doesn't respect my wishes on how to raise my child, then she can stay away. I know it sounds harsh, but you have to do what's best, and they have to understand that you have your own family now, and they have no power in it. They had their own chance at parenthood, however it may have turned out, and now it's your turn. I'm sure if someone would have tried to tell them how to raise their kids, they would've been irritated too. Besides, you don't want your child growing up confused, and not knowing what is right and wrong. He/she needs a strong foundation and a firm moral background to grow and develop from. If he/she sees that you are unsure and weak, then he/she will undoubtedly start to walk all over you! Be firm, and tell them what the stipulations are...don't bargain! This is your child, and you don't HAVE to compromise!
Good luck!
Kat