My Google account been used by another family member ?

I gave to my sister my Gmail account to check some email like 6 months ago. After that I ask her to log out and she said to me that she did. After 6 months I checked my Google account security and I see her phone (iphone) still in the connected devices. So I sent an email to my Google account but her phone didn't receive any notification, I also checked her Gmail app and my Gmail account is not there. So I checked her chrome and I see my Google account there as secondary account. Can someone explain to me what this mean.