Girlfriend sister saw my nude pics?

So me and my girlfriend where over at her sisters place and I when we left I forgot my phone, any way about a hour later when back to get it and when I went on it it showed my photo album have been opened. Now I had made the mistake of leveling some photos I had sent my girlfriend on there of me naked and prittey sure she would have seen them !
What do I do?

Why was she looking?
Should I say anything?
Should I tell my gf?
Girlfriend sister saw my nude pics?
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