Her bad credit cannot affect yours
Adding someone as an authorized user is common practice among family members who want to help each other build credit. It even has a name: Credit Piggybacking.
Appointing a family member as an authorized user on your account has no negative effect on your credit. You are the primary card holder, you are the only one held responsible for paying of any balances. As long as you are responsible – nothing bad can happen to your credit.
However, do keep in mind that your sister has a bad credit, meaning her ability to control her spending is somewhat low. She may run high bills. That may increase your utilization, not to mention that you may end up paying for her spendings.
My advice – appoint her as an authorized user, but DO NOT give her the actual credit card. That way she can enjoy your steady stream of payments and build credit, while you don’t risk her running high bills.