I was just about to start a thread about this! I really like my MIL, but she does grate at times. I've been trying to get to the root of the issue.
I think it's true what PP saying about their role being devalued by accepting that the modern way (aka DIL's way) of doing things is actually sometimes better.
My MIL has been staying with us to help while we have baby twins and a toddler. I can see there are times she wants to advise me but lets it go which is fine. There are times she suggests things that I disagree with, and as long as she doesn't keep pushing this is also fine. The really annoying things are when she clicks her tongue or rolls her eyes rather than just saying something.
Examples include my sticking to government guidelines regarding cheese and alcohol when pregnant. "That wasn't 'in' in my day." But I can see now she would have to accept these days that things she unwittingly did could have seriously hurt her babies, so she's defensive.
I say baby doesn't need baby oil - she sneaks in and slathers it all over him. Then makes a big show of how nice his skin is. It already was perfect. But that's how she did things in the past. I let it go.
What's really annoying is the helping without giving me any credit because I'm younger so she just assumes I'm hopeless. Expecting everyone to sit down at the table for dinner at exactly 5:30pm with a wonderful nutritious meal cooked by me with two newborns is just dreaming. To her she's showing me a template for how things should be. Honestly if she asks me one more time (at 3pm on the dot every day) "have you decided what's for dinner, mum?" when I literally haven't had time to use the loo I'll scream. Every day! And every day I say, "I haven't actually had time to look in the fridge yet but I'll let you know when I do". After weeks of, "yeah no you go eat because I have to change these nappies then pump milk again then get toddler in bed and I'll see you around 9pm" she relaxed the 5:30pm rule a bit. Yesterday she even said, "can you see I'm trying to get you in the habit of thinking about dinner?" I explained in detail why sitting at the table for a meal is for people who don't have newborns and how I managed to get us all fed just fine until the twins arrived, and how we will all be sitting at the table as soon as certain members of the household can even sit, and there is nothing wrong with takeaway for us because toddler has already had his own nutritious meal.., But she is really trying to help.
I feel I could easily be offended and react badly to all the things she says/does or I can just see that it's coming from a place of love and defensiveness and just let it go. I really do let a lot of things go to keep the peace.
This will be another case where things seem very petty, but it's pretty constant and builds up. And I realise this is nothing compared to the actual evil MILs (and DILs).