It depends on the stage of his case. If the settlement agreement hasn't been signed yet, and custody is still being contested, then I would certainly consult your fiancé's attorney before making a decision. If the settlement agreement's been signed, then consult the agreement - if it doesn't prohibit such actions (probably not) then I don't see the harm in moving in. That said, your fiancée is best advise to notify the mother of the move-in - it's a goof faith gesture. For a full assessment, schedule a consultation with a NYC Divorce lawyer.
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Speak with your fiancé and his lawyer about his case strategy. Hard to advise without knowing all the reasons for the delay.
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Without knowing where things stand in the process and without knowing the personalities involved it's hard to be precise. But from a legal standpoint there is no harm in taking either position.
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