I unlocked Nina once, but to reach nusance rank with her would take so much time that I decided to prestige. Exact triggers are unknown for me, but all hobby ranks were at 24 and was pretty far in Hunter, Zoo, cleaning and art.
No idea what the exact trigger is, since she was there when I returned from a 8 hour break.
Reach "Awkward Besties" rank with Elle, she will unlock the last 2 hobbies. Sky diving gives Luck, which is her loved trait. Loved traits give auto affection with the girls, you can see their loved traits at the stats window on the girls.