What should I do?
MIL is generally lovely, though we have fairly opposite personalities, I do my 'best' to get on with her.
We have been out of the country working for a while, and stayed at the PIL house last year for a fortnight. We were invited to leave our excess clothing, to pick up on our next return as we were struggling to fit everything in our suitcases. I left several items of clothes, one of which I had loaned to her when she was caught without - don't want to say what item it was, as family members may well be reading! She commented on how lovely it was at the time.
Fast forward six months and we retuned to visit and pick up our few belongings. I had forgotten exactly what I had left, as it had been some time.
Due to weather, I needed to loan something, and she presented my item to me, saying I could borrow it 
. I did a double take, then realized it was my lovely item, old make up stain in place, zipper tag pulled off as I remembered.
Anyway, to cut a long story short, she clearly thought it was hers. She even removed it from the bedroom in case 'I packed her x by mistake'.
As I was going I mentioned that I thought the X was mine, to hear 'oh no, I bought this at X on holiday. Look, I even have the X and my things are in the pocket' (like, yes, because you've clearly been wearing it form the last few weeks).
Soooooo...... What do I do? I'm really pissed off as this item cost me a lot of money, and was kind of special, in that at the time I had saved up for it.
I don't think she is doing it maliciously, she is just old and mistaken. However, she has clearly taken a firm shine to it and refuses to see she has made a mistake.
If I make a fuss, I look unkind, but at the same time ....it's MINE MINE MINE! I love it and want it back!!!!
It will cost me a lot of money to replace it as it was very expensive.
Do I just leave it and be graceful (while biting my tongue so hard it bleeds) or take issue? What is reasonable/ unreasonable?