I tried killing her with kindess and she took it as a sign of weakness and an invitation to walk all over me. I called her before one of their visits last year and asked what kind of milk they liked to drink.
That's not what people mean by "killing them with kindness."
When she DOES initiate something, you just have a sweet voice and face.
And yes, you should say, in the moment, "I like having the medicines there--it makes my life easier."
Or, just blow her off. Try to get yourself to stop caring what she says or thinks.
She can SAY "you need to move the medicines." Does that FORCE you to move the medicines? No. Just say, "Oh." No more.
And don't move them. What's she going to do about it?
your BIL can tell you to move the furniture around--do you have to do it? No. What's the worst that can happen? He can keep harping on it. And you can keep thinking what a weirdo he is.
You also might develop a catchphrase and repeat it. I know that I suggest this so often that people have started naming it after me, but it can work.
When BIL says, "you should rearrange the furniture," then say--and ONLY this--in a tone of mild surprise, "it's *our* bedroom."
When he says, "yes, but we own half the house," you say "it's *our* bedroom."
When he says, "I'm in construction, I know what I'm talking about," you say, "it's *our* bedroom."
You do not deviate. Not ever. Not even when he says, "Why are you repeating that over and over?" You say, "It's *our* bedroom." Because you are driving home the point that--hey, it's your bedroom, and there's nothing else to talk about.
When SIL says, "You should move the medicines," you say, "they're *my* medicines." Underlying point: not yours, and you shouldn't be saying anything about them.
When BIL sleeps in the living room, just make noise in the kitchen. Don't sweat it. If he complains, say, "I need to start my day." And maybe, "perhaps you should have other sleeping arrangements."
The bigger issues, like furniture, etc., when it comes up, you should say: "we're all equal owners, yes? Would you like to buy us out?"