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In response to Ravanne: I agree with your point in almost all circumstances that involve single parents; however, this is a different situation since this guy appears to be such an irresponsible clod. Since he was "living in a condo with a friend" (and not, notably, with his child) when they met and began dating, and his daughter wasn't even in the everyday picture, the LW wasn't given any reason to suspect that the pattern would change if she became more involved and thus didn't view her role in the relationship as at least partially a stepmother-in-training, nor was there any good reason why she should have done so. If the boyfriend had been living with his daughter and caring for her from the outset and LW realizes four years later that she sort of hates it but wants to keep the man, then yes, the LW is a self-absorbed twat; however, we know from the little info provided that this was not the case.
"...The kids get first dibs on his time, attention, money, etc. Always. A second wife is stepping into an already existing family dynamic." The word you're missing here is should. The kids SHOULD get first dibs on Dad's (or Mom's) time, attention, money, living quarters, and affection, but this kid was not when the girlfriend met the dad and tragically, it doesn't even seem like the daughter's getting first dibs now. Now, perhaps that should have raised a red flag or two for LW, but denial is a powerful thing, and it seemed like no matter how fucked up the guy's relationship with is daughter is, his relationship with the girlfriend is (was) a happy one. And neither of those things, or the short-sightedness and immaturity that kept LW from reading between the lines, make her a bad person.
My point, which may not have been clear, is twofold: A)I find it suspicious that as soon as Dad scores himself a kept woman, presto, his daughter moves back in and the nanny is a thing of the past; this indicates that the bastard is looking for someone on whom to dump his child, preferably someone he doesn't have to pay (aside, of course, from a sweet flat on the beach).
B)LW is not a bad person for not wanting that role. That statement goes tenfold if she didn't know her boyfriend would try to spring it on her given the circumstances of her first three-odd years with him. Had she been a saint she may have unilaterally taken that role for herself, or suggested to her boyfriend that he assume his duties as a father, but alas she is human. Again, I'm guessing the people who were so quick to bash her would have done close to the same. In this case, both the daughter AND the LW are victims of a self-absorbed, lazy, irresponsible shit who should probably never have had children (especially with a mentally ill woman), but who nonetheless did. What LW has going for her is that, unlike the poor daughter, she has a way out. Better yet, she is absolutely within her rights to take it--and she SHOULD.