Your DH won't address this and tells you to 'suck it up', because of guilt.
He doesn't want a blow up and his DD telling him home truths about how he badly let her down.
This may never move forward without her doing that, though. She's carry a lot of stuff that she needs to let out.
She is possibly implicating you, because you came into her life when she was still a child and did nothing useful to help.
Your circumstances are yours, not hers and are very different.
You show a remarkable lack of understanding and unwillingness to not have this out with your DH.
As said, you can't compare your DD with her, your DH helped to provide a stable home and good emotional support for her, whilst not doing the same for his existing DD, that is never easy to get over.
Has your DH even had a conversation about any of this? Going No Contact, or being unable to not have contact with a Parentcwho has an addiction at such a young age as your SDD had to, is massive and I only hope that the Aunt is at least acknowledging it, because, it's passing the two people (you and your DH) who should be there for her, by.