Hi jamesparker001,
Welcome to Apple Support Communities. We understand that you're having an issue with receiving notifications from your sister when she sends a text message. We'd like to help.
Have you ever been able to receive notifications from your sister when she sends a message? Do you receive the text message successfully even though you aren't seeing the notification on the Lock Screen? If your iPhone is unlocked, do you see notifications from your sister?
Have you checked this resource on how to Change notification settings on iPhone? Does that help?
Have you accidentally blocked your sister's contact information? Block phone numbers, contacts, and emails on your iPhone, iPad, or iPod touch Check to see if that is causing this issue.
Let us know how that goes.
Take care!