Not wanting to talk to my Aunt and Uncle anymore

It seems you have been let down by the older generation on all sides. Your aunt and uncle have been weak in putting the onus on you. ('You would have acted out.') What they really mean is that it would have been understandably upsetting for you and therefore upsetting for them too, a difficult topic to broach. They took the easier option of keeping quiet and not rocking the boat. They should have bitten the bullet at some point and told you gently, reassuring you of their love and commitment to you.

Having said all that, they don't sound like bad people, just weak ones. They took you in after all and that's a big deal.

Your mother should have maintained contact with you while you were growing up, even if she wasn't in a position to raise you. She didn't but wants to make amends now. Again, it sounds like she was a weak person, weaker than your aunt and uncle, but again not necessarily bad.

One possibility is to meet the stepson without making any further commitments to meeting your mother. It may bring some closure to hear about your mother from a different perspective. Depending on how you feel after that, you could take things further or not.

The one thing that is clear is that you deserved better.

As the saying goes, friends are the family you choose. Sometimes they can offer enough validation and strength to allow you to be in contact with trickier family members. Only you will know if you are sufficiently supported in that way to make contact.

Honestly, I can see why they made this decision.
It's something that could be hugely damaging, and telling someone information like this right at the start of their teenagers years, or during their teenager years, could change the course of their life's direction. It may not have done. But I can see why they didn't want to take the risk. Arguably yes, they should have come clean when you were an adult, but we don't know how you were mentally or how your life was at the time you became an adult - maybe they were still worried about the impact it might have on you. It's easy to say from an adult point of view 'I deserved to know the truth, I would have been fine' but young brains can be such wildcards and are not known for being able to process things in logical ways.
Your brain is now looking for some way to lash out, and that is very normal. They did what they thought was best, and they (in your eyes) fucked up - but can you honestly say you've never hurt them, either intentionally or unintentionally?
I hope you find peace with this.

I agree that no child should have to feel grateful for being brought up, so I'm a bit uncomfortable with people saying OP is ungrateful.

But I do think that you need to have some empathy for the position your aunt and uncle were in @candragonsbepurple . It must have been a really difficult time for all of you.
It was clumsy of them to say that they thought you'd act out. But basically it sounds as though they were trying to protect you from the hurt that might make you act out.

I suspect that I'd have chickened out of if turning you too. They tried to give you a stable home. Telling you something that would be so upsetting and a further rejection of you by your mother, would be so difficult to do, and risk that stability..

Given you're not handling it well now then your aunt and uncle were right not to tell you.

I can't see they've done anything but try and protect you from someone who was toxic.

I can't see what they've done that is wrong. If your Mum wanted you to know about her remarriage surely it was up to her to tell you? I agree, you sound ungrateful.

You sound so ungrateful and I can completely see what they mean when they say you would act out. Look at your exact reaction here!!

These people raised you when your parents left you and you turn around and this is all you take from their generosity.

Unless there is more to your Aunt and Uncle's behaviour that you haven't said (eg you feeling second best in the home, only grudgingly spending money on you but freely spending money on their own DC, always making you feel like an interloper) I'd give them the benefit of the doubt.

There isn't always a "right" answer, often it's the "least bad" scenario that you try to go with. How hurtful would you have found it to know that your mother chose to have her step children in her life and do for them what she wasn't willing to do for you? How badly might you have felt as a child? They were probably trying to protect you from that.

Being abandoned by your mother is an awful thing to have happened to you. It can also set a blueprint where you "abandon" and cut off others.
If you're aunt and uncle were good people then they didn't do this to cause hurt but rather to avoid it.
It might be a good idea to get some therapy as there's a lot going in here.

Your Aunt and Uncle brought you up and cared for you. They didn't have to. It sounds like they were trying to protect you. Your mother, if anyone, should have been the one to tell you.

This. And wven if they did make a mistake in nit telling you OP does that negate all the years they spent raising you?

Wow.

Firstly- children shouldn’t have to be grateful for being brought up. What happened wasn’t OP’s fault or responsibility. The aunt and uncle chose to raise her. She was a child with no agency.

That said, OP you say you didn’t act out— as a child who’d been abandoned you would have been fragile in all sorts of ways and your aunt and uncle will have worried about how to keep you stable. There will have been times you were devastated by the space in your life when you didn’t do as well as you could have- maybe you went quiet, maybe you got stroppy, maybe you had nightmares. There will have been something.

They will have wanted to protect you.

Your feelings are absolutely valid, and I think it would be ok to tell them you want to take some time to process things. But don’t think in terms of cutting them off for their ‘lies’. Just take some space to process it all and decide if you want contact with your half brothers and birth mum.

Unless they were bad parents to you, keep them in your life.

If I were you I'd actually discuss it a bit more with your aunt and uncle. It could be that there's more of a backstory here. Were they trying to protect you from her in some way? In any case I am not sure they've actively lied? More omitted to inform you of the updates on your mother who abandoned you?
I think your anger is probably with your mother here and I find it odd that this stepson is getting in touch too. He has no particular relationship with you.
As a PP said, this sounds like a very tough emotional situation for you and some counselling may be helpful.

It's a huge thing to take in a niece or nephew as your own. They weren't perfect, but neither is any parent.

I made the same mistake of trying to protect my DDs from something. I really regret it, but my intention was good.
I can only imagine how hard it would be for your aunt and uncle to tell you that your mum had other children, in the knowledge that she continued to not want you. I'd want to spare a child from that, too.

But you've just said you're not interested in your mother and her family. Given that she was so active with men, it's not beyond the realms of possibility that she would have a new family.

Your Aunt and Uncle have done what they could to protect you from your Mothers ways. It's not them you should be directing your anger at.

It was good of your aunt and uncle to step in and raise you. I'm sure with the way your mother was, it was difficult for them to decide what to tell you and what to keep secret.

You need to talk to your Aunt & Uncle - or whichever of them is your mum’s sibling - and tell them you are upset to have found out this way, and do not understand why it is so

But, assuming they treated you kindly , think very carefully before you cut them off. They are not your parents so they may not feel they can forgive you anything . It may be clumsy wording , but I think they might expect you to be grateful for your upbringing .

I think cutting out an aunt and uncle that raised you because they didn't tell you something that would have been hurtful is really ungrateful to be honest.
They did their best for you, and you sound quite immature because you don't seem to have realised yet that no adult is perfect and everyone makes mistakes.

Your mum perhaps made more than most but you are still holding everyone else to an unachievable standard.

You say you were only moody/naughty when sick?
Strange because my teens are the opposite but regardless I doubt your recollection....either way that doesn't stop them thinking not telling you was for the best.

Of course you are hurt, however I wouldn't make any rash decisions on the back of that hurt.
How have your Aunt and Uncle treated you over the years that they stepped into support you as a child?
If they have been kind and decent I would suggest you throw them a little slack.
Perhaps consider getting some therapy to help you navigate all of this.

It was not your Aunt and Uncle's place to tell you that your mother remarried.

Your anger should be towards your mother.

Yes, you may need some time out to process the news but the only person you should be cutting out is your mother.

It's a stepson who wants to get in touch

Your lack of gratitude towards the spoke that raised you is staggering.

Assuming they treated you well growing up and there’s no drip feed. Yes you are unreasonable to cut them out.

There's a whole barrowful of hurt here.

Let Aunt and Uncle know you need some space for a while to process everything.

I think that’s harsh on your Aunt and Uncle. They made a mistake (in your eyes) but does that really outweigh everything else? None of us are perfect and it sounds like they tried to do what was right.

Maybe they felt telling you your mum had remarried & had a new family, when she'd abandoned you, would be a cruel & hurtful thing for you to hear.

It could be as time passed they felt the moment had passed to tell you

If they brought you up in a loving home then going NC seems an over reaction. By all means let them know you're hurt but just remember they probably did what they thought was best

Going NC 'for a while' is a risky move. If they're hurt by you doing it, they may not be there for you when you go back to them

Be an adult. Talk to them

I agree. If this is the only reason you want to cut your aunt and uncle off then thats strange and I don’t understand why you’d do it. Also don’t expect to just cut people off “for a while”, that’s not how healthy relationships work and don’t be surprised if you do that they’ll want to go NC with you, permanently.