I think if the Sister in question admitted it, she'd be turned over to the Black Ships. If she resisted in any way, she'd likely be accused of heresy and witchcraft and then burned.
Mind you, actually being a Sister might make figuring them out as a psyker to be really hard. They hear prophetic voices? Not exactly uncommon for Sisters. Random things happen to their benefit? It's the hand of the Emperor protecting them. Uncontrollable Psychic phenomenon? Sisters of Battle have a ton of anti-psyker equipment that might end up suppressing that. Daemonic Possession? Again, Sisters of Battle routinely cover themselves in holy wards and the like. Even as a psyker, they'd be harder to possess then the average person.