How would you feel if your mom tried to abort you?

You're just going to have to live with this because it's not your place to tell the surviving twin.

All families have secrets but this is a big one which should be kept unless you aunt wants to reveal it further.

I'm not sure if I understand your actual question: in that situation I wouldn't feel anything because I wouldn't exist. If you mean finding out later on, I have no idea, but possibly grateful I exist