That's a sad scenario you lay out there Guy. I have no doubt at all she feels objectified and a sense of obligation to get cut up whenever her sister needs it. Tuvix had that same sense of obligation and certainly everyone else objectified him. When Tuvix said no, they said screw you and cut him up anyways. If the girl said no, they'd probably be pissed as all hell but I doubt they'd drag her to the hospital, strap her to the bed and cut away. That would be a criminal act of the first order. And it doesn't even involve -killing- the girl. Heck, if the girl said kill me, take my heart for my sister... that would be illegal and a doctor would lose his license if he did that to say nothing of criminal charges.
Tuvix committing suicide.... that would be a selfish act... but in a world where he's objectified and his life isn't of any more value than to get two other people back.... I could see the desperation that suicide is the only thing that -he- can choose.