You think you know people. Especially when your related to them by blood. It hurts when you find you don’t know them at all. That they are complete strangers to you. What hurts more than that is when no one knows you and you fucking live with them. Share the same DNA with them. It’s heart-breaking.
Best advice is to get over it quick as you can and learn from it, nobody is your friend in this life, just look after yourself.