In the case of the OP's sister she practices with 2 different guns shooting one handed so speed and accuracy might not be an issue in her case, especially if she is familiar with a variety of guns and can shoot either equally well. What concerns me is does she have any martial arts training because having a knife does not a knife fighter make. Same side of the coin is does she practice with that knife in her hand because holding an object in the left hand changes a whole mess of dynamics.
Range practice is good but situational practice is a whole nother ball game. Take cover and call for help instead of trying to clear the house is the best option but if she is in a situation where she can't take cover or is in the situation of having to provide safety of others in the house then it behooves her to be practiced with either hand, learn to shoot from behind either side of a barricade and to know how to use a knife.
Maybe learning that will convince her to go two handed on the gun. From the sounds of it though she sounds like a lady who will not take kindly to intruders and could be a menace to boogermen who enter her domain.
Good intentions will always be pleaded for any assumption of power. The Constitution was made to guard the people against the dangers of good intentions. There are men in all ages who mean to govern will, but they mean to govern. They promise to be good masters, but they mean to be masters.