I once framed my sister for something because she broke my phone.
Why would you frame her for breaking it?
She accidentally broke mine, so I smashed hers to get her back.
You should confess to framing her for breaking the phone. It is unkind to break your sister's things, especially in retaliation for an accident. It would be good to make amends with your sister.
Maybe I'll confess, but only if I have to...
You really should confess since you did something cruel by breaking her phone. If you confess, it will make you feel better and will help your relationship with her. Don't you want to have a good relationship with your sister?