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It's because some of us have been around the block a time or two, have learned that personality traits such as groping your sister are very deeply ingrained and exceedingly difficult if not impossible to change, and have learned exactly how these situations tend to play out (which is to say: badly, for all involved). Our experience tells us not to fish but to cut bait PRONTO. By doing so, so much pain and heartache will be avoided.
Our experience also informs us that, yes: a single mother raising two small children by herself is a very long, very hard row to hoe, but it can be done, and - this is most important - it's preferable to being married to a man who will repeatedly grope our sister.
As I said before, I'm in favor of divorcing the sister, too, being of the opinion (likewise based upon hard-earned experience) that one needs to expunge such toxic people from one's life, but I'm sure that will be even more contentious a position than merely advocating divorcing the creep.