I don't try to answer all their questions, honestly. I don't feel it's my job to make other people feel secure about my decisions.
It would feel good if everyone was happy with our choices, but sadly you can't make all the people happy all the time.
Next time she asks you that, just respond, "We are very busy, and we are very happy. Thank you for asking." Period. End of explanantion.
(I've read a lot of your MIL posts, and I feel for you)