We all want the best for our families and our society, but these types of choices are between the hypothetical sister and her husband, not the sister, husband, government, and me.
The logical conclusion of your line of thinking is that the government can regulate your brother-in-law's diet and exercise regimin, lest he die early and leave your sister a destitute burden on society.
Once you begin to walk down that proverbial road, there is nothing the government can not and should not regulate.
A red herring. There is nothing that governnment cannot regulate or at least attempt to, in human behavior. One only has to change the Constitution, for example, to take away constitutional "rights."
"Slippery slope" is fallacious here as in most places in my humble o.
I think communist regimes like China and North Korea do this based on such a reason of "obvious" government interest.