Howdy sir,
It is really hard to diagnose things without seeing them actually happening in real time. But the next time you go out with your sister, you should fire a few rounds out of your 75B first. How does the brass eject for you when you shoot it? It may be that your recoil spring has grown weak and needs to be replaced with a new one. It could be as simple a fix as that.
But if the brass is not hitting you, and ejecting a decent distance, then I would agree with those who have suggested your sister is likely limp wristing the pistol. When she does that, it robs some of the recoil energy the gun needs to operate properly, and is likely why the brass hits her.
Also, what type of load are you using when she shoots the pistol. If it is a very light load, and she does limp wrist a bit, that will contribute to the brass hitting her in my view. If using a light load, you might let her try standard loads and see how the brass ejects.
Everyone tends to have their own shooting system that works best for them. But all of the gun schools I have attended over the last 4 decades or so as a police firearms instructor have emphasized keeping both elbows locked. So I have to disagree with the suggestion to have her not lock her elbows. In my view if she keeps both elbows fully locked, that tends to give her a stronger grip as well.
As I said, it is really hard to diagnose things over the net, but unless the recoil spring is worn and too light, limp wristing is the most likely cause of this situation. Unless the grips are just super large for her hands, I have a hard time visualizing how a long DA trigger pull could be causing this problem. But another suggestion is to let her try shooting a mag or two all SA, and see what the brass does then as well. Just a thought.
Good luck,
twoguns
