You don’t have to enter the SB line from what I can tell. You approach the CM, briefly explain that you can’t wait, and your family will enter the SB queue and you’ll be able to find a place more suitable to wait per your needs, and then you’ll be given access to the LL once your family is near the merge point. Or if you’re solo, you’ll be given a return time to then access the LL.
At least that’s how I understand it. They’re accommodating people who cannot enter standby as well as people who can (or sometimes can), but then have to duck out due to an emergency. Nothing says you are required to give it your best shot at waiting and see what happens.