Leolady, I know entirely how you feel.
I have invested money into my kitchen equipment. Now, I won't say that it's A LOT of money, but, money is money. I have some professional-grade equipment, and a lot of tools, "gadgets", etc. I tend to get finicky about how my stuff is used. Many times, I've come home and realized that my MIL has been "mishandling" and "misusing" my tools, or just using them in a manner I don't think is appropriate. I'll usually give her a warning or two about how I feel, but she does as she pleases. After a few "infractions" on my feelings about my equipment, I will go and hide the piece for awhile.
I realize that this is mostly just my "regimented training" in the culinary world, and that my MIL was brought up extremely poor and had to "make do" with EVERYTHING. I still just don't like to see my tools used in a manner that I don't want them used.
Neither one of us are willing to compromise on these issues, so it just boils down to a simmering stale-mate of me waiting until she decides that she wants to live somewhere else.