I find it hard to believe that it wasn't ban evade if he took a screenshot of your email on your account. He has access to your account and from I've seen you and your sister haven't really played at the same time until that day.
I suggest you to change the password and probably the email aswell on your steam account if you don't want it to happen again.

I find it hard to believe that it wasn't ban evade if he took a screenshot of your email on your account. He has access to your account and from I've seen you and your sister haven't really played at the same time until that day. I suggest you to change the password and probably the email aswell on your steam account if you don't want it to happen again.