AITA for not wanting this ?

That's a big decision, it should be absolutely have been a joint decision

You would not be an AH if you said you won't care for the child, he has to.

You both are . He’s definitely in the wrong for not consulting you whatsoever about adopting and taking in and raising his nephew but I also think it’s a AH move to say he’s fully your husbands responsibility and you won’t help care for a innocent child . How is it not fair for the child to be adopted by their uncle tho ? I’m curious u said he’s adopted his nephew don’t they need u to sign and adopt him too since your married ?

Hell to the absolute no. My mom was sort of in this situation. I have two cousins who spent a lot of their childhood in foster care, and my parents took care of one of these kids on a temporary basis. They both have fairly severe FAS and lifelong disabilities. It was a struggle. My dad's family thought that he and my mom should have adopted her, but she was not prepared to do it, and they never forgave her for it. You have to stand your ground. No one can force you to raise a child. You can choose to, if you feel you can handle it - but it is not fair to you or your family, or your life to expect you to raise a child just because someone else thinks you should. It sucks that the child has to suffer for it. But that can't be the reason for you to turn over your entire life.