iMessage recognized when it shouldn’t be

I used to have a named group chat with my siblings who all had iPhones. Messages sent were iMessages.

Now, 2 of my siblings no longer use iPhones, but the messages still send as iMessages, so 2 of my siblings don’t receive them. I tried entering in all the phone numbers separately again hoping they would be recognized as non-apple users, but that didn’t work. I also changed their phone numbers to “mobile” instead of “iPhone” in their contacts.

Is there a way to make that group specifically sms messages or a way to flag those numbers as non-iPhones?

I am looking for a solution to text all of my siblings together where they all receive the messages for this mix of iPhone/android users. Thanks.