So don't give her the treat until she sits nicely for a few seconds, I know what you mean, it seems like its killing htem just to sit there for one second. But that is another exercise you have to do with her. Teach her patience. Since she is food motivated, that makes your job a million times easier!! Carry your treats around with you, while you are doing laundry or something and get her to look at you at weird times. Do this is only with Nala, have your other two put away when ever you are doing training.