I'm one of the posters who replied when you wrote about this topic in January. Back then I suggested setting a date, and when the date passes, giving everything you don't want to Goodwill. I suggested letting all of the relatives know about the deadline so that it would be harder for FIL to whine about you not giving him a chance to get his stuff. (I also told a story from our family about a relative who wouldn't get her stuff and then lied for years that she didn't have a chance to get it before it was discarded.)
I'm guessing now that while you are very frustrated, you just do not have the ____ (determination? bravery?) to go ahead and stick to a deadline. I understand that it is probably very difficult to stand up to your FIL after so many years of obeying him.
So, I'm going to build on a suggestion someone else made in this thread. This suggestion will cost some money up front that you probably will never get back. But I suggest that it may well be worth it to you to solve your problem.
I suggest that you rent a storage space - climate controlled if need be - for one month. Then hire a local mover to move the organ and whatever else you do not want into the storage space. Then send your FIL the storage space rental agreement and tell him that paying the rent after the first month is up to him. If he wants to keep the organ in storage, fine, it is already there and he can pay for it to stay there. If he doesn't want to pay storage, he can go ahead and give the organ to the church or sell it - or he can abandon it at the storage place by not paying any more rent. Whatever he decides to do - it will be his decision and his problem, not yours any longer.
Yes, I predict your FIL will be upset and will likely say mean things. I'm suggesting to you that maybe it will be worth it in the long run to put up with him being upset in order to be rid of his stuff. The advantage of this plan is that it gets his stuff out of your house once and for all.
I don't know what this would cost where you live. I'm guessing in the neighborhood of $300 to $500 where I live (but I don't know, it is just a guess). I know that is a lot of money, but even if you have to save up for a few months to be able to do this, it puts you in control of your own house and it gets the problem items out of your house.
Good luck with whatever you decide to do (and please come back and let us know).