Name change in group thread
You have to be very careful in this situation.
Any one of the participants in the group can have an address book entry that is wrong and send the messages incorrectly when adjusting group membership.
There is zero need for foul play here. First apply Hanlon's razor (Occam’s razor applied to people and technology) and substitute “stupidity” with “people are busy and don’t meticulously track which AppleID they use to sign in to their devices. People want to share purchases and don’t set up proper family sharing and separate their accounts between Messages and the App Store or effectively and regularly audit who logs in to their accounts.”
Compound that confusion on three to six devices per person and three to six people, and you can end up with an intractable mess when one name maps to a different AppleID that has multiple SMS and emails attached for delivery.
Back to your specific situation, are you sure the name of your sister’s children maps to a Message account or SMS number they access?