My family has had a lot of troubles, including mental health, other health issues such as dementia and also addiction.
This is a recipe for a really fucked up family dynamic. Everything you write screams obligation and co-dependency. It's not healthy, and most sensible people would want to distance themselves for reasons of self-care.
my parents... are aging and my mum especially craves company.
What has your mum done to seek out company / friendships? Your mum isn't your responsibility - she's a grown adult who has seemingly chosen not to establish friendships outside the family.
My parents are eccentric and possibly have undiagnosed mental health issues
Have they sought support or a diagnosis? Or do they just rely on the two of you? Their mental health is not your or your sister's responsibility, and you shouldn't act as though it is.
What is she actually giving to the family? I don't think anything.
Have you ever asked yourself what the family is giving her? Given the MH and addiction issues, do you think your family contributes positively to her life in any meaningful way? If not, why are you expecting her to give and get little in return?
Also, what does your family contribute positively to your life? If not, why haven't you drawn boundaries like your sister has?
Your sister is not responsible for your parent's mental health or their addiction issues. It is also not her fault that you are so enmeshed in the family dynamic that you don't look outside it for fulfilment and happiness.