I don't know how to do a proper quote but this seems to be the heart of why it is bothering you:

"There's also the bit where my brother tagged my other brother in the photo, and the two of them are besties, and I really just feel like neither of them can be bothered most of the time to remember they even have a sister. And if I did disappear, it would be no skin of either of their backs. I'm stuck with feeling either insignificant or inconvenient to them most of the time. And I don't know how to stop caring about this, so I stay in this stupid 12-year-old frame of mind. And I wonder if it would just be easier to disappear after all - not in the permanent sense, but in the connection to family sense; dropping the line and all that."

These seem like real feelings, not "just" an episode. It seems that the picture reminded you that you feel "left out" of sibling bonding. Do you have a therapist you can discuss these issues with? In the meantime, I recommend doing something productive that helps you release the bad feelings, like something creative and cathartic such as art or singing or a physical release like exercise. HUGS!!