My brother has horribly sweaty hands and he ruins my guitar strings among other things?

I taught my little brother to play guitar and he has 3 of his own guitars. I have many guitars, but was once a professional. He likes to sneak and play my guitars when he visits my house, ruining the strings on his very first play. I have designated nice guitars for him to play, he also brings his own but he spent the night the other day, snuck into my studio (which I keep locked, but he knows where the keys are) and gunked up a telecaster and a les paul. I know how to clean the strings, but it's never quite the same. So I have to change them to achieve the same tone for my recordings. I love my brother and don't want to be a jerk. He's emotionally fragile but he's cost me hundreds of dollars in strings and he knows he's not supposed to do this. He's also costs us thousands of dollars because he's inadvertently done things like damage our pool, damaged the laminate, equipment, etc. He doesn't work so he can't pay for things. He's almost 23 so it's not a matter of age and still lives with our dad. What is the proper way to deal with this since he won't listen to reason? He is a very sweet guy and my 2 1/2 year old daughter loves him. My wife feels sorry for him, as I do, yet he's expensive and time consuming. He supposedly has an IQ of 165 but has severe learning disabilities. Thanks for any advice.