You can apply for a B-2 visa extension if you file before your current status expires. Make sure you include medical documents showing your sister’s condition and your role in caregiving.

It’s possible to extend your stay by filing Form I-539. Since you're helping your sister with a health-related issue, that's a strong reason to include in your application. Just be sure to submit it on time, ideally at least 45 days before your I-94 expires.

Helping a sick relative is often considered a valid reason for extending a visitor visa. You’ll need to show proof, like her hospital records and a letter explaining your role. While USCIS can take months to respond, staying while your application is pending is generally allowed. Just don't let your current I-94 expire before applying. If you wait too long, you could fall out of status, even with good intentions.