@ArdoCycle
I guess I probably could be hard work, but I don’t think it’s unreasonable to set this small boundary. Why can’t they walk together. I don’t think it is kind when they know it causes anxiety.
I'm going to try and be gentle.
I have a relative with diagnosed anxiety. It's an awful condition and you have my sympathy.
That said, my experience was that my relative used her anxiety as a crutch to maintain control of any situation, regardless of the impact on anyone else.
Everyone was expected to confirm to their "demands" (some perfectly reasonable others bonkers) so as not to increase their mental anxiety.
The expectations became intolerable impacting major family events (such as demands on how someone else's wedding was located/planned/seated etc etc).
Upshot is everyone stoped enabling the behaviour. Life got better.
You need to separate your legitimate issues from your anxiety.
It's reasonable to expect that if your MIL takes your twins for a walk, that's she's back when she says she will be or texts to explain why she might be late (late meaning 5/10 mins). If she won't do that the she doesn't get to take them out.
It's reasonable that your MIL should not be buying bottles/formula for BF babies. If she does this again, she can't be trusted to supervise the twins alone.
It's not reasonable to say because she has stents that she is not fit to take the children for a walk (my MIL had a stent fitted after a heart attack).
It's not reasonable to say you are ok with your parents looking after the twins but not your MIL because of your anxiety. It is ok to say it's because your parents follow your parenting rules (I'm assuming they do) and she doesn't).
On the positive side she is obviously wanting to be engaged as a grandmother. We see so many threads where posters are so sad that they get no engagement at all.
My own experience is that all sets of grandparents have greatly enriched my DS/DSD's lives.
Their collective love and experience has been beneficial. But we've not always agreed - but that's ok, because ultimately as a parent I get the last word - but I do listen to their views (and sometimes discard them rapidly
).
What I'm trying to say is that you need to separate your MH issues from the legitimate concerns you have about practical care of your twins re:MIL.
Good luck 