I wouldn't worry at all about what your MIL thinks or says. I'm quite sure my MIL doesn't think I can mother one, let alone two. But I have learnt not to care about it (mind you, it has taken time). Now I accept that she will always have a different way of doing things, and that she will never see agree with my ways.

Please don't let things that others say upset you. I am sure that your MIL was trying to be helpful rather than critical, but either way, you know yourself better than she does