If I did not have siblings or other family to besides my parents, idk, I can't picture how I would fill the void they fill in me. I'd probably be more creative. I've made some very close friends in my life, so I would at least have that.
The thought of them leaving, I know will be hard. Being alone is something I've always felt, but actually being alone is another story. I would cope and survive, because that's my only option. Who knows what happens when we die, wishful thinking is nice, but spend that time thinking about how you can maximize time you have left, for yourself and others. Maybe I procreated and I have to make sure my offspring have the tools to survive in this world, with or without me.
The people I respected most in the world would be gone, determined to become and even better version than them, I would ask myself, what would they do in this situation? And I know I would fail in some capacity.