What would make someone you don't snitch on call you a snitch, even though that person has told on you?

Who knows? Depends on the context of the situation. “Siblings” doesn’t tell us anything.

An unconscious desire to be punished for past wrongs.

Guilty conscience…?

People may not be able to see the situation realistically.

Projection.

Trying to cover their own butts. . Right or wrong, fair or not, the authorities usually think the one who called for help is the party in the right.

Unless its the cops, then everyone is just fucked.

Projection of course.