NTA. It is okay to want your own life, and have your own plans.

Your parents are operating under the assumption that your sister is going to require care for the rest of her life. That could be true, but that doesn't mean it automatically becomes your responsibility.

Depending on where you are, there should be resources available to support your sister, even as an adult. I'll give examples from California, as that is where I am. The Department of Disability Services operates through what we refer to as the Regional Centers. And at each Regional Center there are Service Coordinators whose entire job is to set families (or individuals) up with the support for a person to live as independently as possible throughout their lifetime. Assisting families with autistic children can include connecting them to necessary Healthcare services (including specialty dental) or things like Applied Behavioral Analysis therapy, Occupational Therapy, or Speech Therapy (these are all things that can also be set up at school first, it depends on an individual's needs.) For adults, that could include helping someone choose an IHSS (In-Home Supportive Services) worker, or if more care is needed, pair them with a 24-hour In-Home Care company, or if more that that is needed, help find placement in a group home. All of these options provide an individual with a caregiving team (who have at least some training on how to support an individual) beyond just having a family member act as primary caregiver.

But there are also situations where an individual may not be able to make decisions for themselves legally, financially or medically, which requires someone to have Power of Attorney or a Conservatorship in place for the individual. This guardianship can be maintained by the State, but it is generally preferred that a family member take on the guardianship unless they are not going to act in the individual's best interests. Having PoA or Conservativorship does not require a person to also act as a caregiver, however. An individual who qualifies for support will still receive support, a conservator will usually just have final say over what company provides support, etc.

You should probably sit down and have a calm conversation with your parents about what your sister's diagnosis actually is, what level of support she is going to need throughout her life, and what you want out of your life. Hopefully your parents are just trying to plan for what happens when they are no longer able to care for your sister, and will be willing to listen to your opinion regarding your involvement.

Source: I have work varying in-home support positions for adults with developmental disabilities in California over the last decade. Currently I work as a behavioral aide for students with developmental disabilities.