This tricky for me to answer. Partly because I'm not female and don't have that perspective, other than second-hand. And partly because I'm concerned over getting the right balance between reassuring you and making you too complacent. But -
Granada and Sevilla are large urban centres and, of course, both have less salubrious districts. In the case of Sevilla, notoriously Las 3000 Viviendas, which you absolutely should avoid, day or night. But these are not areas the average tourist (or local) is likely to visit, even accidentally. They won't appear in guidebooks or be on a walking route to anywhere that is. The places you will be are not dangerous. It's certainly safe to walk around the small, winding streets of the historic centres - you'll find lots of others doing the same, day and night.
The more likely hazard in either city centre is theft - pick-pocketing, stealing of unattended bags. Whilst petty crime isn't out of control, it is still prevalent enough to make it wise to always take particular care of your belongings (and leave things you don't need with you back at the hotel).
On the other hand, physical assaults on tourists are very rare. And if "night" means late evening, you'll find lots of other people around in the city centres at 10 or 11, and later.
I'm not a single woman, but personally, I wouldn't have thought the Albaicin is dangerous after dark and you'll find it's main streets and plazas busy; but walking around deserted side-streets could be creepy and there is some bag-snatching apparently. So perhaps stick to the busier parts. Also personally, I'd avoid Sacramonte alone late at night, especially avoid wandering into the rough-ground, cave parts.
You must do what makes you comfortable, but twice daily checking-in seems excessively prudent for Spain. More importantly, what will your cousins do if you fail to check-in? Do they have a realistic plan?
There are no guarantees in life, but I'm 99.99% sure you will leave Spain thinking physical safety was never a concern during your visit.