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...is not that the LW does not want to be a step-mother, but that she has allowed herself to get into a four year relationship with a man with a child. And allowed herself to get involved with a man who is apparently a terrible, neglectful father.
At least she has faced the reality of the situation before getting married and is ready to remove herself from a role in which she doesn't belong.
I think it's unfair to condemn her simply for not being willing to be a step-mother though. I am close to her age, and I do not want to be a step-mother either, but I resolve this problem by not dating men with children to being with.