That family ending is what's gonna kill her. You're not taking a random curse, you're sacrificing some of your life force to give your sister time so you can find her a cure for her sickness, and you become sterile in the process.
If you don't find her a cure, and instead force her to create a baby, what do you think is going to happen? If the sickness was weak enough that they can make a baby and still be around to raise it, why would you sacrifice your lifeforce to give her more time? Nah you're sentencing her to death, you don't continue to the grand adventure afterward.
The popular idea on incest is if you don't make a baby it's harmless. Dev doesn't want to be known as pro incest babymaking.