Help: I discovered spy camera pen in my bathroom!

Hello there,
I am dealing with a very serious problem and your help would be much appreciated.
I live in the same house with my husband, my sister and her husband (my brother in law). I recently discovered a spy pen camera (with a usb recorder in it) in our bathroom when I was about to take a shower. We put it on the PC and saw on camera my brother in law which proves it is his own device and he was actually using this pen to spy on me and see me naked.

We were absolutely shocked as we have been leaving together for the past few years and we don’t know where this behaviour starts and where it stops. Is it the first time? Has it happened before? Etc.

My brother in law is quite clever and smart, however at the same time looks quite sensitive and psychologically immature and has a look which sometimes reminds you that of a serial killer or someone psychologically unstable. He has lost his father and he is still pretty mad with him(never forgave him) as because of him the family lost their fortune. After the father’s death, he was on pills for a long time as he had depression. Over the last 4 years that I know him though and till recently he looked almost normal. I have noticed him staring at me sometimes for 1-2 seconds (staring my body) and sometimes I used to feel uncomfortable but I was always thinking I am over reacting so I ignored it, however I used to be very careful in terms of what I was wearing at home so as to not look attractive.

One day, I came back home early and I saw him pleasuring himself but I pretended I didn’t. He thought I might have seen him though and he came to check on me straight away (which I found quite cheeky as he looked normal and not ashamed) and I pretended everything was normal and he did the same like I never saw him. I thought that this was normal (even though I personally don’t approve these kind of stuff but I thought if I would tell my sister she would be hurt and overreact so I did not say anything.) A year ago we found a mobile device (he was hiding it from us) and when we asked him what this is he said, someone lost it and he took it. We thought the right thing would be to take it to the police but he thought he deserved to keep it. We did not like this kind of behaviour and my sister asked him to return it which he did. He would not have told us about it if we haven’t heard the ringtone coming from inside the wardrobe. Sometimes he is hiding food and sweets he buys for himself, as my sister disapproves certain types of food as she is worried about his health. Sometimes my sister and brother in law leave together so that he can give her a lift and he pretends to go to the gym (that’s what he tells her). Instead, he comes home straight away spending hours in front of the PC doing other stuff. It seems like he is lying a lot –that’s my point.

Combining certain types of behaviour together, especially after the spy pen, we (me and my husband) decided to go through his stuff to see whether we would find anything similar as we started worrying about what type of person he is and we did not know how to handle the situation. From the way of his upbringing we could feel empathetic and believe that he might be acting as a silly and immature teenager who just needs to be told off and forced to get back in line after giving him a good scare. We thought perhaps we would have a chat with him and see if there would be a chance to help him if it is an addiction (as everyday he watches porn and pleasures himself –we have proof for that but not sure if that is normal or not at the age of 45 when it happens so often)and/or a psychological issue as at the time we thought it might be the first and only time he used the spy pen camera. Unfortunately, after going through his stuff, we did find out spy camera sunglasses , voice recorders and another camera/video recorder as well as a dvd with my name on it (it was erased already though). We are really shocked and we think that he might be watching us for a long time now and we don’t know when and where this stops.

We feel the right thing to do is to get my sister involved and make her aware. However, we are really facing a dilemma here as we are scared that he might already have found out that we know (and so we don't know if we are “safe”). On the other hand, my sister who is married to him is a lovely person who has no idea what is happening. She has been really struggling in her life to find the right person to spend her life with and now she thinks she has found the perfect guy and revealing something like that would rock her world as she is very sensitive and has no other friends, she is very attached to him, plus she is 40 years old and not like the usual independent women of our society.

She is still quite innocent (like a little girl in some ways) and in a way psychologically immature and I am afraid if she finds out what kind of person sleeps next to her (especially when he has been trying to spy on us and perhaps has videos with her sister naked etc) she might even fall into depression and never be able to trust anyone again, lose her self esteem etc. At the same time my parents live in a small village where everyone knows each other and they would feel devastated and embarrassed if everyone finds out that their daughter is getting a divorce. I am also really concerned about their health as this would make them be in so much pain and worry about their daughter's future. Can you please advise? I am really desperate as I am keeping something so big from my sister. Shall I keep the secret to avoid hurting her and my family and avoid destroying her happiness? But then, do we let her live a lie her whole life. Also, is this an addiction? Can it go even further? How should we handle it? What is the next step if we don’t do anything else. Who knows what else we would find out with a more thorough search through his dozens of usb sticks and hard disks.. Please please help!!