I wrote a fake letter posing as my mother and gave it to my sister.
Is your family having issues?
Well my mom can't write properly so I do it for her.
Even if your intentions were good, forging someone else's writing is extremely wrong. Your sister believes that letter was from your mother and would be crushed if she found out you wrote it, and your mother would be too. You should be honest with both of them and tell them what you did.
I'm not even interested in getting through my sister. All I wanted to do was to sabotage my mother's reputation.
That seems really shady. You know you will get caught one day and get in trouble. If you keep this up as an adult forgery is a crime.