To be fair, shit happens.
[zombie-brand-mindblown]
If you don't explicitly say "I'm back! My brother was playing for me while I took a dump, what happened while I was gone?" - how would anybody know? You were never logged off during the switch, and were most likely running on the same machine you're usually on.
By the sound of it, it seems that you're not sharing your account information, and you're not asking for help for the purpose of boosting. Sharing account information with anyone, including family members, puts your account at risk. You don't know how someone else is going to react, and their actions on your account can't be shrugged off as "it wasn't me, my brother did it" when something goes sideways.
While sharing accounts can lead to punishments leading all the way up to a permanent ban based on past history and how serious the offense is, a single act such as this without any prior history wouldn't be anywhere near enough to result in a ban.