iMessage Issue

I was in a text conversation with my daughter this evening and it appears my step-daughter was able to read them. Background: my step-daughter had been on my carrier plan for more than 10 years. In February last year, she transferred her number to her husband's carrier plan. She kept the phone and number since it was tied to her work situation. I have verified her number is off my account.

I use iCloud for all my Apple devices (MacBook Pro; iPad Air; iPhone). Is it possible she was able to read the texts between my daughter and I? If so, how can I stop it?

Thanks.