Apply for the EU family-member residence card (tarjeta de familiar de ciudadano de la Unión, form EX-19) for household members without an EU passport — apply in person within 3 months of entry; the application receipt proves legal residence until the card arrives

If your household mixes passports — one of you EU or Spanish, the other not — the non-EU family member doesn't ride a visa: once in Spain they apply for the EU family-member residence card (form EX-19), in person, within 3 months of entry. The receipt you're handed on filing already proves legal residence while the card is processed. Bring apostilled, translated proof of the family relationship (marriage or birth certificates) and evidence of the EU or Spanish family member's own status.