wendygrace, i'm also certain it doesnt *have* to be the way you described. i'm sorry your sister was in stirrups, didn't have gravity or the help of an upright (open) pelvis. it is hard to watch, i know.
others have had great responses already, just wanted to add that for dd's birth (birth center, CNM), w/ a very gentle & hands-off mw & nurse, i felt very unsure of myself when it was time to push.
i was changing positions, fully dilated, trying to remain very relaxed -- but i was sure i "Didn't know how to push!" and kept begging my mw for instructions. Which muscles? How? How much? And she just kept answering, "But you *are* doing it! The baby's moving down, here's the head."
And i would say, "No, i'm not doing anything. i don't know how to push." then she was crowning, and her heartrate didnt recover btwn ctx - and then i did give one major full body effort breath-held push and out she came.
My point being that the whole time i thought i wasn't "pushing," my uterus was working, my baby was descending. I credit this to the deep relaxation and time i spent in the water, and position changes. And my brilliant daughter, of course, wjho *did* know what she was doing even when i didn't, lol!