Account Sharing?

It's selectively enforced, meaning you won't usually get in trouble for just letting somebody play on your account a few times. (Mind the word "usually", they can ban you for it either way). The real issues are boosting and account security.

Boosting: Should be self-explanatory. Account Security: If Riot has reason to believe you cannot uphold account security due to sharing problems (typically when multiple people try to "recover" their account from being "hacked"), they'll just close it permanently.

Edit: Oh right, and bought accounts are also usually closed, when it's discovered that they were bought. Don't know if levelling up accounts for money is also punished, but since those services usually also pose a risk to account security and most of the time even use bots, it's definitely likely to get you banned.

Ya but Riot doesn't care about it.

Only if the person you shared with plays ranked.

I let my brother play to try out new champs/champs he doesn't have all the time.