this is a decision his mom should have made. that person is not you. I think they overreacted a bit he should know eventually but they are right to be angry

You are not his mom. You are the woman who birthed him. Your sister, who you gave custody to has raised him and is his mom.

YDI, not because you gave him up, because you went about this the wrong way. You should have talked with your sister first about a potential plan to tell him when he's older. Six is far too young to tell him, and you've probably scarred him for life. What were you hoping to achieve? You may have given birth to him, but legally and morally you are not his mother. Your sister is, and she should have been consulted.

yeah ur out of line, and if u don't see that seek a professional

That's heavy, I don't support lying to children, feels like almost a right to know who your parents are. Something that big feels like it needs to have a thoughtful, coordinated, delicate reveal.

The kid needs to know the true circumstances of his birth if only to be able to provide his doctor with accurate medical history. So your sister shouldn't hide the truth from him. I'm not even sure that's why you told him though? More importantly, you shouldn't usurp her role like that. She's the one who has been raising the child, sacrificing her career maybe, getting up countless times in the middle of the night, wiping dirty bums and many other thankless tasks involved in childcare, and you don't deserve to call yourself his mother simply because of biology. If you wanted the kid to know, you should have talked about it with your sister, preferably before even giving him up to her. If this didn't occur to you at the time, or if you've changed your mind (which you are allowed to do), you should have at least discussed it with your sister before revealing the biological facts to an unsuspecting child.

your sister is his legal mother. it was her place to decide if and when he found out. 6 is far too young to have this thrown on him and that was not your place. you will never be his mother

As painful as it is, you should realize it's his mom's decision when to tell him that. What you did was wrong, you probably should apologize, promise never to raise it again without her permission and offer to go to family counseling together.