I'm going to accuse my sister of buying fake Instagram followers
Why do you think that her followers are fake?
I saw her doing something suspicious on the laptop.
You should stop interfering in what does not affect you. Being labeled as a person that accuses without proof can carry a very negative connotation, as someone solely interested in the negative things that is affecting a person's life.
No one cares, my sister is not going to ruin her life just because of a few stupid followers.
There is no good reason to bring it up then. You should not accuse people of things that you have no proof of.