Advice on course of action..

Although by no means serious in the grand scheme of things, I have a situation I'm not sure how to move forward with. Speaking to friends is tough as I believe they might infur identities no matter how cryptic I was and I'm not wanting to try to rationalise and give my thoughts at the moment in case I'm being too soft/tough etc with my current views and way I should approach things. As such I'll give just the facts. I've discovered by chance (due to the fact that mac laptop's sync iMessages sent from other devices) that my brother in law has sent himself 3 intimate pictures of my partner and I, along with about 10 non intimate pictures of just my partner to himself using my phone whilst

I must have hit the word limit and not realised as my message seems to have been cut off? basically he was pretending to change music and has sent himself some pictures from my phone, tried to delete evidence, he is married to my partners sister and they have a 8 week old child and is due to be my best man in August. This happened last weekend but I've just spotted it today whilst using laptop. I went into more detail originally but think thats enough, clearly affects a few different relationships as well as the embarrassment/awkward/pride angle of it all etc Any advice on a method of approach appreciated.