I think you can just turn off iMessage on the mother's account... Pretty sure that's what I did when I had just an iPad a few years ago and was facing a simila

I think you can just turn off iMessage on the mother's account... Pretty sure that's what I did when I had just an iPad a few years ago and was facing a similar problem. It's really dumb and stupid and annoying to explain to people why I didn't get their message.