OP, it wasn't unreasonable to ask her at all.
I've been depressed for the best part of 15 years. Sometimes I can't even get myself out of bed. Actually dressing and brushing my teeth was beyond my ability. But there have been other times I've been much more able to get out and about.
The past four years have been a particularly difficult time, and the thing I've struggled with most, is simply not knowing how I'm going to be from one day to the next. There have been many, many times when I've been unable to do something planned.
It might be (not saying it is) that this is what your MIL is struggling with. The commitment of two days per week might simply be frightening.
When I've committed to doing something, even something small, I'll usually feel anxious for a few days before hand. I need routine - getting up, going to work, doing the school run, collapsing in a heap. And this is during my good times. During my bad times, that's not even possible. If there's something extra, that needs planning. Sometimes it needs valium.
So in your position, I might be able to say 'I might be able to help, but won't know until the actual day.' But I wouldn't be able to commit to say, Tuesdays and Thursdays.
After the first couple of times, I might be able to do more, but there still might be a crisis and I'd end up letting you down, and I'd then feel worse and want to bury myself somewhere...
You see how it goes.
I'm not saying this is how your MIL is - it's just one way that depression can take a person. But in that sense, having depression might well rule out being able to commit to twice a week.