Just thought I'd add my experience to the mix.
Have you thought about the pressure that new mums are under these days with the advent of social media? Everyone posting pictures of their perfect parenting moments on Facebook, Snapchat etc.
She will probably have gone to an antenatal group where it all kicks off with expectations for the perfect (limited intervention) birth, to whose baby sleeps through the night first, baby led weaning experiences, first to crawl/walk etc. Then there's all the baby books written by the 'experts' containing all sorts of conflicting advice. (Bloody Gina feckin' Ford!) Besides unhelpful comments from strangers and 'well meaning' relatives.
I have a 7yr DS and a 3yr DGS.
Obviously, I was an older mum and reasonably confident about my parenting abilities but my poor DIL, despite being a highly educated (3 degrees) professional woman, really struggled to accept that good enough was just fine. She did seem overly anxious and controlling initially (cleaning the toddler's teeth after every meal and snack!) and followed a very rigid routine. We live in different countries so I wasn't around to give practical support.
However, they visited us a few months ago and I was so pleased at the change in DIL now DGS is 3.5yrs. She was very relaxed, teeth cleaning was just twice a day and even allowed him to share chocolate and sweet treats with my 7yr DS. She was back to her old lovely self and the visit was great fun. My step-son seems much happier too (although too attached to his phone at times).
My only bit of advice is to occasionally tell DIL what a great mum she is. I think my DIL needed to hear this to know that I was on her side, supportive of her and had no intention of criticising her parenting choices. From what I hear, I'm far less critical than her own DM so she sees me as an ally, I think.