First of all, congratulations on being able to be honest with yourself. I study Family Business dynamics and the challenge with them is the larger they get, the smaller and more stifled one feels - a bit of a paradox.

As you may know, vision is everything for a business. One person may have wanted to persist with Apple as a Computer company, but now they are a technology company. Two entirely different realities.

This makes our first choice clear:
we do not want a major role in this business.

That takes us to our second choice - would we let it go completely? This depends on the following:

How likely is our individual success?
What would happen after we left?
Would the business survive?
What would happen to our relationship with our sibling?

If you'd like a further discussion on a call, we can talk about the a) validity of your personal career choice b) the line of conversation to have with your sister and c) the exact role you should carve out within the joint business for yourself.