So good news! My annoying BiL sent Mr. Ivriniel a picture on his phone showing that the hallway that we need to take the piano down has been cleared.
Then this evening we went to my FiL's place, and found out why the Annoying In Laws are storing the piano at their overstuffed house. It's so ridiculous, it's unbelievable.
Apparently ABiL had approached my FiL and asked if they could store the piano at his place. Weirdly, he never said it was a piano, just that it was a large piece of furniture with sentimental value to ASiL. My FiL agreed, as long as they were careful and didn't scratch his new floors.
Anyways, then ABiL posted to Facebook looking for someone to help move a piano. My FiL's Girlfriend responded to the post something to the effect of "Oooh! You can store it at my place! I'll look after it, (and play it for you.)" Now my FiL knows that his GF used to play the piano, and misses not having one, and thought that would be a really nice solution to the storage problem, so he chimed in something to the effect of "Seconded! That would be a great idea."
When the Annoying Inlaws got their panties in a bunch. They interpreted his Facebook comment as him rescinding his offer to store the piano, and sent him a nasty email saying to the effect of:
"What is this crap about storing the piano at your GF's place? You said family can rely on each other, but clearly we can't rely on you!"
I swear, they start every day looking for ways to make life more complicated than it needs to be.