system
Given the following post, respond as if you're commenting on Reddit:
human
[Back To The Future] Did Marty's siblings witness their bodies fading away as they did in the photograph?

After Marty stops his parents from getting together the first clue that his past is changing is an image of his older brother fading away in a photograph. Later, when his brother and sister have both vanished, we see Marty's hand fading away before his eyes. Does that mean that in 1985 both his brother and sister experienced the horror of slowly vanishing? Did other people see this? What about other changes to newspapers and photos? Was everyone in 1985 Hill Valley freaking out, witnessing things and people around them slowing vanishing or changing?
gpt
No, they just didn't exist anymore.

In Back to the Future, people fade from existence once they no longer have the ability to prevent whatever time change that altered their past.  That's why Marty is the last to go -- he's the one who is actually in 1955.

For Marty's older siblings, they were living as normal until the moment Marty went back.  At that point, their fate is out of their hands.  Whatever Marty does in 1955 will affect them before they're even born.  They don't have a time machine, they don't have any way to influence Marty or change the events of the past.  The exact alterations to the photograph are some quirk of time travel that we don't really understand, but Marty's brother wasn't walking around without a head.  Regardless, if Marty succeeds, then his brother and sister will have always existed.  If he fails in getting his parents together, then his brother and sister will have never existed.

Think about it like this -- Marty sticks around until the pivotal moment of the Enchantment Under the Sea dance.  As long as George kisses Lorraine while "Earth Angel" is playing, Marty's okay.  He isn't going to disappear as long as he has the ability to make that happen.  He only starts to fade out when George is gonna miss his chance.  But for his brother and sister, they have zero ability to make 1955 George kiss Lorraine.  That's why they start to fade as soon as Marty pushed George out from in front of the car.

In 2015, Old Biff travels back to give a sports almanac to his 1955 self.  Old Biff experiences no problems until he returns to 2015.  At that point, he starts having health issues.  They get worse as he moves away from the Delorean.  That's because his ability to go back to 1955 and fix his own past is going away.  He doesn't fade out until Marty and Doc get back in the time machine and leave.  At that point, Old Biff's ability to repair his past is gone completely, and he disappears.

The changes Old Biff made don't affect Doc and Marty, because they've still got a time machine.  It's not too late for them to fix the changes.