NTA as this was clearly accidental. That said, soft YTA to you and your family if not addressed.

I may be downvoted but I think you were wrong to hide this: the truth always comes out. Dependent on how long it takes and how, the sense of betrayal can be too big to repair. I understand why your sister and family wished to hide this but it simply cannot be hidden. If anything, you never know how, when it may come out so risk losing all control of the narrative and the potential trauma.

Now it’s done, best address it so, as a family, you control the narrative. Get some external advice (professional? ie. a therapist?) but frame it as a story both of hardship and sacrifice, yes. But ultimately a story of accountability, growth, love, and second chances.

Done well, this can become a very good teaching moment that prepares your niece for the realities of life but also reassures her she has a loving caring family she can rely on and trust to support and guide her in life. Also, if done well, it may help your sister (and your mother?) come to terms with her obvious feelings of guilt and shame and focus on her own growth so she can be the best version of herself for herself and her daughter.

May I add? You express no bitterness for the path forced on you. This internet stranger thinks you a very good person. One that makes the world a better place.
That said, the fact it came up in an argument: are you truly that ok about this or is it an unresolved issue? Please do check in on yourself and make sure you look after yourself too!

Wishing the best to you, your niece, and your family.

YTA. I’m sorry you went through this op. No child should be dropping out of school to raise other little children the adults at home should be raising. I know you said your mom picked up more work but the onus shouldn’t have been on you, your sister should have stepped up or put the baby up for adoption. This is your mom and sisters fault 100%. I don’t know that telling a little girl her mother didn’t want her was the best move though. I say this as someone whose own mother said straight to my face once when I was a kid she never wanted me. It really stings. You shouldn’t have said that in front your neice. I think that puts you in AH territory. 

NTA. But your mother and your sister are, it also seems that your sister is the golden daughter because clearly your mother does not force her to return while she made the responsibility fall on you, you are not the bad one but if I were you I would look for a job and if possible later I would resume my studies, the truth is I would recommend you break up with your mother and your sister, they are clearly bad people, too ungrateful and they do not care about you, by the way, your niece’s stepfather knows the truth regarding the abandonment of your sister?

>It’s been a family agreement to forget this all happened

ESH, Everyone Sucks Here.

This should never have been a family secret at all. Plenty of teen parents start with a rough patch, and your sister apparently got her act together and turned it around. Your niece could have had that explained once she was past toddler age and able to basically understand.

You chose to weaponize this information because you’re bitter about your own choices to step up and help your mother. But the person you are hurting more is your innocent niece, not your sister.

Wow. So mom was cool with you sacrificing your entire future to save your sister? Now mom is angry you aren’t still playing the dutiful son who still does exactly what they are told? I can give your sis some leeway as she messed up at 17. Mom was supposed to be the adult in this situation and her best response was to use her 15 yo son to cover for her sister.

Yeah, you’ve got a serious mother problem. NTA.

Nta. You didn’t intend for your neice to hear. And your sister was WAY out of line. Secrets are dangerous. They should have had that child in therapy when her mother took her back from the only caregiver she ever knew.

Dont even get me started on your mom. Shame on her for putting you in this position to begin with. Your education and future should have been priority.

YTA. She’s a child she’s innocent in all this and there’s no way this revelation won’t affect her negatively moving forward. I’m sorry you had to give up your last years as a kid/first few years of young adulthood. That’s rough and nobody should have to do that. However that is 100% on your mother and your sister. Be mad at them all you want but don’t involve your niece.

ESH.

You absolutely should not have said that in front of your niece. She did not deserve to find out like that; in trying to hurt your sister, you hurt her too and that’s not on.

I agree that this couldn’t be kept a secret forever, but your niece deserved to be told properly, and not treated as collateral damage in an argument between you and your sister.

YTA – You’re still blaming your sister all these years later. What she did sucked, but she was young and scared and by all accounts she has turned her life around.

Your mother asked you to drop out and YOU made the choice not to go back. You can go back to school at any point. Even today. Stop blaming your sister for your choices.

Op your mom and sister suck. You are not the AH and it’s never too late to go to school but it will be expensive. You can start doing certificates in your area of interest and see what you need to do to get your ged. Some colleges will accept you without a high school diploma and scholarships exist for all people.

NTA if you don’t want the smoke don’t start a fucking fire. She’s looking down on someone who knows her DEEPEST DARKEST SECRET and is just counting on their decency to protect her.

Run. Her. Down.

Get this all taken care of and out in the open. If her and your mom don’t like it well… fuck em.

NTA, but it does feel like resentment. Now that it’s out in the open, hopefully you can move on and make decisions that affect your own life now. If your mom and sister keeps bringing you down, be like your sis and bounce out of state for a couple of years. Clearly, your mom was okay with that.

NTA because you didn’t intentionally say it in front of your niece. I’m not sure that your niece will forget the comment but, like it or not, you need to honor your sister’s wish. There will come a time when you can have a conversation with your niece. That time is not now.

Yeah, you are an arsehole. But so is your sister. Bringing this up in argument is beyond the pale & you did it to score points. Now your niece knows something she’s too you to handle & it hasn’t solved anything between you & your sister. ESH. 

Yh for all ur niece knows ur her primary parent, she essentially became ur daughter. Bio mum has no right to take her back unless there was harm being done to the child.She is the female version of a sperm donor atp. NTA

YTA- you could have easily said, you have absolutely no room to judge me, and you 100 percent know why. And gave her a LOOK. And privately yelled at her. But to hurt a child for the sake of being right, is sad.

N T A. Unfortunately the past doesn’t always stay in the past. This is the consequences of your sisters actions.

If anyone has any “Explaining” to do your neice, that would be your sister.

Of course you are. I don’t care what your background is or your trauma or thinking was. It was utterly indefensible.

There’s no backpedaling from this to make it better.

Your niece is 11 now if I’m adding it up correctly? I think it’s best to tell her the entire story in as gentle a way as possible. She won’t forget about it.

NTA Secrets are never good for anyone, and she overheard, it’s not like you told her on purpose out of pettiness. Besides, your sister is the real asshole.

You’re not the AH, by a long shot. Mistakes happen, but your niece deserves to know and your sister deserves to have consequences for her actions.

YTA. You have valid baggage, but you need to keep that amongst the adults. You just dropped a bomb on your innocent niece. Not even remotely okay.

Soft YTA, because that’s not how your niece should have been told. I do understand your frustration, though. Your sister is definitely TA.

ESH. Your sister is obviously the biggest asshole here. But you shouldn’t have said that near your niece.