Etiquette for negotiating signing bonus

I have received a job offer but no concrete numbers yet. The HR person has asked me "if I wished to disclose for the purposes of calculating a recruitment incentive if I'm married, have kids and what address I would be realocating from".

I am not married yet but my long term girlfriend would be following me there and job prospects for her are not great in that area (however she can still do some freelance work online). Is this relevent at all? Do I mention my soon to be wife would be coming with me? What is their reasoning when factoring all that?

I appreciate any insight or advice you guys have to offer me.

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