My MIL reacted pretty much the same way when we announced pregnancy #3. I was surprised since she had been over the moon about the first two and is absolutely the most maternal person that I've ever met. I think a lot of her reason for the lack of enthusiasm was just that she had 3 herself, though the youngest was quite a bit younger than the older two, and recognized that it was going to be hard. Our oldest is not an easy kid and she was worried about how hard it would be for us.
Aside from that initial less-than-enthusiastic response, she really hasn't made many comments and our 3rd is now 4 years old

I would probably let your FIL's email go since they have now each expressed themselves once on the subject. If they make any more comments, that would be the time for your husband to ask them to keep any negativity to themselves.