I (36f) found a folder of photos that involve my husband's (38m) fetish.. Problem is, they are his sister (23f)

I'm sorry if this is jumbled and if anything makes no sense.. I'm so sleep deprived at this point and stressed out.. So I apologize.
I've been with David for 9 years and married for 5. We are very happy.. And I wouldn't change a thing. Early on my husband admitted he has a "tanning fetish". He just goes crazy over tans, tan lines, and he said "seeing me tan" but I believe he just meant seeing girls tan...

This hasn't ever been an issue, I definitely didn't judge him or anything.. I was okay with it.. And on occasion I would tan for his enjoyment and things were great..
Last month for Christmas my entire family went to Florida, including his sister obviously. Needless to say, we spent plenty of time on the beach and it was a great vacation basically.

Last night while he was working, I decided to check our computer for some of the photos from said vacation. I was struggling but when I did come across them, there was a few folders inside of that folder. I thought maybe they were edits or something.

So I open one, and it is 50+ photos of his sister in her bikini doing various things, but also, her tanning. How did he take these without me knowing? No idea. I'm extremely hurt and I feel disgusted.
I need some advice on how to handle this..its wrong on so many levels. He's never given me a reason to think he'd be a creep either

Tl;dr; husband has a tanning fetish. Went to Florida with family for Christmas and found pictures he took of his sister in her bikini and tanning. How do I go about this?