My first thought is god this sounds familar, second thought is that if you have a receipt for the barn tell her if you can not use it for its intended purpose (that is if she knew why you were building the barn, which I am guessing you told her when this all started) that you want the money for the barn. You may want to find legal help for this. Your can chose who you marry you can not chose your inlaws. And they do not always play nice.
From my experience this is not going to get better. We live currently in a similar situation, on in laws property. So everything we do is a beg for it type deal. Yet we pay all the bills, up keep, taxes, ect instead of rent. Every time something happens we are expected to just take care of it, and we have to ask permission every time we buy an animal before hand. If not we get a very rude lecture about how we live on there land. Thankfully we are moving in the spring and if you have that option at anytime I would say take your animals and run!