Scheduling an event without inviting someone after inviting them once

Hello WildWeeder,

Thank you for reaching out to Apple Support Communities.

You would only need to select the names of the people you want to invite as outlined here:

"Invite others to an event

You can invite people to an event you’ve scheduled.

1. Tap the event, then tap Edit near the top of the screen.
2. Tap Invitees.
3. Enter the names or email addresses of people you want to invite, or tap  to select Contacts.
4. Note: For you to send a calendar invitation to a contact, they must have an email address. See Add and use contact information on iPhone.
5. When you’re finished, tap Done.

If you don’t want to be notified when someone declines a meeting, go to Settings  > Calendar, then turn off Show Invitee Declines."

Additional details can be found here: Send invitations in Calendar on iPhone

If the issue persists, can you provide a few more details? Are you using an old invite that previously had your sister? If so, does the issue persist if you create a whole new invite?

Cheers!