Email going to wrong person although I am using the correct address

I use two emails for my sister. The gmail address is private between her and me. Three times when I replied to her gmail email, my reply went to the email address rogers.com that she shares with her husband. I have no idea why this is happening. I cut and pasted my last reply to her and put it in a fresh gmail and it still went to the joint rogers.com email address.
Can anyone help me?
Cheers
Kathryn

Three times when I replied to her gmail email, my reply went to the email address rogers.com that she shares with her husband. I have no idea why this is happening

Could be she has an eg: automatic forwarding email address setup in her Gmail account to go to Rogers.com email address.

Go to her Gmail online mailbox and click Settings / All Settings / Forwarding POP/IMAP and check if there is.any Rogers.com email address setup.

Apart from that, if no automatic forwarding setup in her Gmail, then create a new test email to her Gmail address (instead of replying) and see if she gets it.

Also check your eM Client sent box, and see if (when you have replied to her) “if the sent emails show her Gmail address” or a Rogers.com email address.