Being family has nothing to do with it. OP saved her friend from getting married under false pretenses. It's absolutely terrible to coerce someone to marry you by lying about something as important as having kids.
Tell them if they want to get paid for babysitting, they won't get a separate allowance. Pay them the average babysitting rate for your area and only for the hours that you're working. They can try that out and see if they prefer that over just getting a generous allowance.
NTJ. Husband is definitely TJ for not telling his wife. Lots of folks are saying you should have known to inform the wife too. As someone who is ARo, though, why would a relationship dynamic like that occur to you? I mean, that's part of why you're ARo, that stuff doesn't occur to you. And that's OK. There's nothing wrong with that. It was his job to know that his wife should at least be informed. The wife is TJ for thinking she had any say about a child that was not biologically hers, simply because they share DNA with her own child. That is utterly ridiculous. Her anger at not being told is understandable. The husband may have known she would be demanding and that's why he didn't tell her. He may have decided that helping his friend was more important than his wife's feelings. That's not OP's problem, honestly. Especially when legal rights had already been established by both parties that were actually involved.
YWNBTJ, however, I do think you need to consider what you would gain from doing so? Will this make your work environment more stressful with her? Is that worth it? I think you should make a point to get to know her better. Maybe actually spend some time with her outside of work. Try to find out her motivation for copying you so closely. If it's because of social awkwardness/lack of awareness and admiri g your style, you can gently lead her towards finding her own style, even if it's still sort of similar to yours. Help her discover her own "things" to incorporate into her style. If she wants to write about it, have her contribute on *your* blog. You could could potentially create a lucrative business for yourself if she just wants to be taken under your wing, so to speak. Don't make her your enemy. Teach her how to be more uniquely stylish.