Since you are in good terms with your mother in law, why not just say her 'mom, I know you are always here for me and I appreciate it so much that whenever the baby cry, you help to carry her and let me take a rest. But I would like to learn handling the baby on my own as this is part of learning experience as a new mom. If I can't handle, then I will ask of your help'. 'I hope you dont't take it wrong'. And then make sure you smile!