if your sister had you password she wouldn't change it.
no family member would go through the trouble of trying to play your games when your not around when she could just as well use your old pass with you.
what happened was of these 2 things;
1) you were attempting to use some 3rd party cheat programs, and they logged your password
2) you clicked on spammy emails and messages that said they were steam staff or claimed to give you cheats
basically, its all your fault and you know it. don't play your stories here