I think a lot of it depends on Sara's player, and how "vulnerable" she wants the character to be.  If she just wants to be a normal human, then that's fine, but if not, perhaps a certain resistance/immunity to other mutant powers might be possible.  After all, a lot of characters have immunities.  Relatives are frequently immune to each other's powers, so there is something of a canon possibility for one to be immune.  Or maybe her mutant power is just a resistance to mutant energy projection (like Cyclops' blasts, Banshee's screams, etc)