in English (and most Western languages) we generally refer to people by their first name, unless there is a larger hierarchical gap or it is in a professional/business setting. For example often when talking to your direct superior if you are working closely together, especially in smaller groups it is common to be on first name basis and within a family (or family-equivalent, which I guess is the fact for the characters) you generally use just first names or for parents mom/dad or other informal variations of that, sometimes also sis/bro for siblings but that is less common.
Calling someone by cousin, especially regular and not just as a reaction to something or when introducing them, is super uncommon in my personal experience.
On singular/plural: When you refer to a single specific item you use singular ("SS could see the soul as well" (don't have the game open RN so don't recall the text 1:1; meant the scene at the beginning when you meet her); => singular as you refer to that specific soul that is standing there), if you refer to multiple items or something non-specific you use plural (cats and dogs can see souls; => you mean they can see them, not that there is a specific soul right now they are looking at)
For example if you had a soul being surrounded by a handful of cats and dogs then "the cats and dogs could see the soul"; the "the" before both "cats and dogs" as well as "soul" indicate you mean specific cats and dogs, as well as a specific soul.
Another example is at the very start when starting the game the notice about using free-use music which you end with "the update" which is the same grammatical error, as it would refer to a specific update. (The context here as well as in most cases as far as I played - which wasn't too far, admittedly, makes it pretty clear what you meant, though.)
If the deepL translation is unclear at points tell me and I'll try to rephrase it.