I'm not saying you are responsible (you didn't starve it and it was not your snake), BUT: You knew she'd let an animal die the same way before, presumably as did DB. She doesn't feed the family dog and you knew would just leave it (as presumably did DB, again). You knew she didn't even seem capable of maintaining her OWN living environment or properly taking care of herself, as terrible smells were common from her room (apparently akin to that of rotting animals) and it's filthy...then, you saw signs she was not feeding the snake, and you smelt the corpse smell.
What other signs might you have been expecting that something might be or had gone wrong? What about your dynamics or aspects of your life that made you and DB over look the signs? I don't say that accusingly, I am sure there are reasons to do with how you all view each other ect.
It's not like there were no signs this might happen and she's never done anything like this before, though, and it's not come out of the blue. There were signs before she got the pet of where this might end. This was ignored or over looked by two older, apparently mentally healthy and sensible adults (you and DB), until it was too late.
Maybe you were too busy with your own life to see it, or intervene and only now looking back it is clear, but to just keep saying she was an adult so why would you have thought...? You've already told us why someone who knows her as well as you and DB might think she'd starve or had starved it. Otherwise, you would not think she starved it when it was discovered dead, would you?
I would worry about someone living that way looking after a pot plant, let alone an animal, and that's without all the back story of past animal neglect (the guinea pig and dog).
There is something wrong in the sibling dynamic here that you both seemed to think she was suitable to care for the snake without being checked up on, and are not worried about her own living conditions or how her upbringing might be affecting her, seeming to think you 'had it worse' so she can't have been as impacted or impacted in a different way, ans she's 18 now so must be capable due to age. You even sound a bit resentful of her, having apparently been more protected than you.
She should have never been given the snake by DB and someone should be asking what is going on with her, as there are signs she is struggling. It's not normal to be so lazy you live in that level of filth and mess, even without the neglect of her pet. This is more than laziness. I think you all need help and potentially counselling regarding your family dynamics, as well as directed specifically at her behaviour. It might help you and her with confronting her issues, and how you all feel about how you grew up.