iMessage/text message

Group messages only work if the recipients use iMessage. But it is possible to send iMessages to email addresses. Check whether it is her email address that you added as a recipient in the group message.

Alternatively, you can actively unsubscribe her phone number from iMessages.

This can be done from her iPhone if she is still signed in, or online here:

Deregister and Turn Off iMessage - Apple Support

Yes that is correct. But the recipient who does not use iMessages will not be able to see the replies in the group, or reply to the group. Have you tried Deregistering her iMessage phone number?

Once you've done that, try removing your sister from your family group and add her back - it shouldn't suggest sending as iMessages.

Oh yes, I understand perfectly well. Unless your first message is worded incorrectly.

I summarize. Your sister has got a Samsung Galaxy phone, and therefore cannot send or receive iMessage. When you send a message to her, it is sent as a normal green SMS, its fine. But when you send messages inside your family group, it sends iMessages to everyone, including your sister. But because she uses a Samsung, she doesn't receive them.

The solution (may be) to deregister her phone number from iMessages via the link I wrote in my first reply. Solution number two (may be) to remove your sister from the group - AND add her again, in order to register that your sister should NOT receive the message as iMessages.