I don't think the cheating tag is warranted. Sure, her son may feel weird about it, should he find out, but there's no mention of a father anywhere in the picture.
The very first chapter is not on FAKKU, as it was published before FAKKU started publishing X-Eros. It can be found out there on the internet, but as far as I know, only in Japanese.
I don't see why they would not be able to get married. Traditionally, if a Japanese patriarch's son was a dolt, they'd adopt a more capable young man, and if possible, marry him to a daughter, to ensure the bloodline continues.
While not traditional, or all that common, it's perfectly legal for stepsiblings to marry here in Denmark, as they aren't actually related. (On another note, while uncommon, it's also perfectly legal to marry your first cousin. In fact, it is throughout most of the world. Dunno why particularly Americans treat it as such an outlandish thing.)
Well, it's not that it sours me - I'm not THAT old-fashioned; I live in Denmark, after all - it's just a thing my Asperger's brain can't help wondering about. >_>