My brother in law is doing weird shit to me

This has been going on for a little while now, but I'm tired of it and need to do something.
About a year ago I was over my sister's. I stayed over because I live 3 hours away and we had a few drinks that night and ended up staying later than normal. Her husband whom I thought was a nice guy for years came home late, didn't realize I was staying over prior to that.
The next morning I went in to shower, got out and went to get dressed. I noticed my clothes weren't folded the same way they were when I got into the shower. Also there looked to be wetness on my panties. I was kind of embarrassed so I didn't want to bring that up. Could have just been me, but I remembered exactly folding my cloths a certain way like I always do.
So I go downstairs and my brother in law tells me he's making coffee for us all. Great. He's in the kitchen and I told him thanks I gotta grab my phone (which was upstairs) but then saw something on the TV that captured my attention so I stayed downstairs. I happen to look over and I see my brother in law in a bent position motioning his arm quickly at something. I looked away not thinking much of it and turned back a few moments later and he stops the movement and I see him putting a coffee cup on the counter. Then I realized what he was doing. He was whacking off into a coffee cup. It was a maroon looking one. So I run upstairs and grab my phone. My sister meets me on my way back down. There stands my brother in law saying coffee is ready. He turns and goes into the kitchen. I started to walk fast to attempt to grab the non maroon cup but he was already at the counter. There I saw a white cup and two maroon cups.
He immediately grabbed the white cup and took a big gulp. Then my sister walked up and grabbed a cup and handed me the other. I wanted to say something about what I saw and was almost positive what he was doing. So the fucked up thing was my sister was drinking her coffee and started to complain he gave her a cup that wasn't washed or the creamer was spoiled. She dumped her cup so I did the same. I could see a kind of angered look on his face. It's then when I said I had a long drive and would pick up coffee on my ride home and I left shortly afterwards.
The final straw was they came over a couple weeks after that. My sister wanted to run to the store to get some things so I said I would go. He was there and said he wasn't feeling up for it and would wait. I didn't want to leave him alone in my house. So before I left I ran upstairs, grabbed some panties and placed them on my bed a certain way. I took a picture of how I left it. Then I opened my drawers just enough to put a hairclip inside and closed it on the clip so when I opened it again the clip would fall. I ran downstairs and my sister and I left.
When we got back about an hour later I said I had to go to the bathroom and went to my room. I looked at my panties on the bed. Totally moved, but almost in the same position. I looked at the picture I took and took another of it afterwards. Then I went to my dresser drawers. I went to open the top drawer and when I did the clip didn't fall. It was already at the bottom of the drawer. That meant someone else moved it. I looked at my clothes in the drawer and some were wrinkled and it was untidy. Not how I leave my stuff. I was totally weirded out and felt violated.
I told one of my girlfriend's what had happened and she laughed and said he (my brother in law) is an attractive looking guy and should be happy he does that! I couldn't believe my ears. I don't think she believes me, but it's true.
I'd love to leave a camera out while he's at my house or something to see what he does, but with my luck he would probably find it and claim I was spying on him because I have the hots for him or something.
At this point I just want to avoid him, but that means avoiding my sister. He always wants to come with her to visit and whenever we go out despite me being single. I already tried to use the third person date excuse and told my sister I needed sister time, but there he is last time she pulled up. Driving up with her. Fucking jerk.
What should I do?????? Thanks.