1) ask sister why she thinks it is that important. Listen to her arguments. Then explain why some of them are wrong - if they indeed are. 95% chances that once upon a time someone, probably SLP, scared her up crazy about aspiration precautions. Explain her what changed since patient got G tube. If necessary, draw or even show her Xray to see where tube sits (in the fundus). Explain her what is done to prevent aspiration. Accent that nothing in the world, in fact, prevents it 100% - including positioning.
2). Explain her that this neck's position is uncomfortable and eventually causes pain. If she doesn't believe, ask her to sit like that while talking with you for 20 min.
3). make serious effort to give sister something else to hold onto. This behavior is a classic example of "holding onto that straw", typically demonstrated by low-educated family members in attempt to regain some semblance of control and "do something". Do not try to teach them seeing the whole picture, etc., as they are not able to do it. Re-address their control impulses by asking, for example, for help/cheering during PT/OT, doing ROM, entertaining, reminescence therapy, etc. as appropriate.
4) drooling must be really significant in order to cause dehydration, but if it bothers anybody for some reason, there is Scop patch for that, unless contraindicated.