How to deal with MIL - impossible?

Copy this question, remove the first sentence and redirect it to her in an email, you have nothing to lose. There could be lots of reasons for the women's message (perhaps she resents the distance between her and her son and may be irrationally be jealous that you are with him) she may even regret sending it but not know how to put things right. Failing that just send her weekly emails telling her how your lives are progressing, pictures and updates about the baby, do not attempt to judge her or expect replies but at least you are keeping the door open for her and you will have no need to feel guilty about your actions, if you cut your loses and ignore her it could lead to resentment between you and your partner later on.