I'm a daugther in law and I did (maybe still do a little) have a lack of confidence around the in laws - I want them to love me and think I'm great and a compentent mother even if I do things differently to how they brought up my husband, ultimately I want their approval - I sure at times I have been a nightmare (although not conciously, I know I used to get very defensive) With my mum and dad I know I have unconditional love and understanding even if I say something stupid - I'm not sure with my inlaws and I'm looking for signs they even like me sometimes as its so important they do, I'm often on my guard so I don't say the wrong thing.
I think just as you are trying to get used to being a mother in law she is trying to work out where she stands as a daugther in law and maybe (as all kids do) find out what the boundaries are.
It has also taken me a long time to work out that when I was learning how to be a mother, my in laws and parents where learning to be grandparents and sometimes got it wrong too.
Maybe she is worried you'll disapprove of her parenting and is trying to justify her choices to you already? The sigma of bad relationships with inlaws also doesn't help, we are brought up to believe that we won't get on.
The break through with my mother in law was one day just saying I was finding it hard as a parent as no one ever told you if your were doing a good job, I noticed she started saying things like how happy ds was, commenting on his ability to do things like 'hasn't he got good fine motor skills' and not 'he's a late walker isn't he - X is the same age and already walking' (like she used to) or if I said 'he's been a handful' she'd say dh was just the same as a boy and I shouldn't be so hard on my self and she'd soon tell me if we needed to call in super nanny
as I've become more relaxed in their company (and I've been with dh for 14 years, married for almost 7 and have 3 children!) I find it easier to let their annoying comments slide and be more confident around them, I guess I feel I have less to prove, my relationship with dh is great and we are our own family unit.
Not sure if thats much help but only to say if she starts off on some critism make some none commital sound and change the subject - think of what she says that annoys you - does it really matter? is she just insecure in your company? is it a lack of confidence? what do you want from her in terms of understanding and acceptance? is it that she's asking for the same thing from you but is being a 'child' about it?