Sister-in-law Keeps Being Pregnant At Me
Dear Vexed,
What a mess! Your sister-in-law does seem very impolite, among other unfortunate qualities. You mention that she remains unmarried, but I should hope that by now she's at least secured a regular suitor. If so, you or someone you trust in your family should speak to her kindly but gently about the importance of proposing, so that the fact of her children make more social sense to outsiders, to say nothing of those in your current community, who are most certainly gossiping.
If she does not have a regular suitor however, it may be time to consider having a serious (but gentle) conversation with heads of your household about her behavior, even before addressing this new pregnancy with her. Having a child while unmarried is one thing, but to do so without any obvious awareness of the consequences her own family's reputation goes far beyond rudeness, to say nothing of doing it twice.
For the good of your family legacy, I encourage you to talk with your household, and eventually with her, about the consequences of her reckless actions. If she can't be brought to bear, some loving financial or social pressure should be strategically applied, for her own good, and even more than that, for the good of both her children.
In the meantime, I advise doing as much as you can for her child. She'll clearly need a model of good, upright femininity, since her mother can't be trusted to provide it at the moment. I hope for your sake, the child's, and especially her own, that she mends her wicked ways.