Guy's Behavior

I think my boyfriend is a bit too close with his cousins. Am I just overthinking?

So, my boyfriend has always had a close relationship with his cousins, which didn't bother me at first but I found out he had matching bracelets with his cousin which I personally think is a bit odd plus he lied and said it wasn't a matching bracelet then later on i found out he had it with his cousin. I was thrown off but decided to put it aside for a while. Later on, he went on a trip to California and stayed with his other female cousin for about a month or so. He has sent me pictures where they were always wearing matching outfits and were way too close to each other in the pictures. I didn't know what to think so I asked him and I got yelled at "Because I'm crazy to assume that something is going on between them". I felt embarrassed then let it go again. After he was back, I randomly saw that he was saving bikini pictures of her in snap and the conversations were something like "Don't cry looking at the pictures, I know you miss me" and just odd way off texting a family member. I am also close with my cousins but I have a distance because I see them as my own siblings. After I saw that, I got mad at him and I asked him why he would save something like this. His excuse was "I wanted to see what she sent so I saved plus they always dress like that in California" ... I genuinely don't know what to think here. Am I overthinking this or is something really off about their relationship?

I think my boyfriend is a bit too close with his cousins. Am I just overthinking?
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California