Stop and do not do anything for awhile!!!! I can see why this would be upsetting to you. That it probably makes you want to share what seems momentous with someone else, but you found the pictures, and you will have to bear the burden.
Think about yourself first. If you go around telling people this, it will be you who gets hurt. No one is going to thank you for it.
Also, what can you “tell?“You really do not know what is going on.
The one person you need to talk to, but not in an accusing way, is your sister. Before you start, take her aside and say, “I have something I need to ask you about. I need you to hear me out and listen to me without getting upset.” When she agrees to that, quietly and calmly tell her how you stumbled across the pictures.
If she reacts strongly, remind her that she said she would hear you out. Tell her if she wants to rag on you for seeing the pictures at the end of your discussion she can, but first there are more important things to talk about.
Tell her you care about her, and you would like her to tell you about the pictures. Tell her that if her sexual orientation is gay or bi, that that is her business and that you are OK with that.
Then see what she says.
If she is gay or bi, then ask her what her plans are regarding John. Do not tell her about him popping the Q. At most, say “I think he is pretty serious about you so you may have to be careful not to hurt him.” Then stay out of it. It is their relationship.
Also, say nothing to his brother.
You may want to mention the girl she was kissing. Depending on what your sister says about that, you might mention there is a big maturity gap between 13 and 17, and it is not probably the best girl love interest for her.
I hope things work out. You are not directly involved, and it needs to stay that way.
Find a safe person to talk to (a friend in a different state who does not know any of the other parties, for example) if you are just bursting. Even then, don’t name names.