In Miracle Nikki it was hinted that Nikki have a sister. In Hello Nikki Let’s Beauty Up she makes appearances. Too bad that the game is no longer available.
In current game, Nikki’s sister name is Yoyo. Nina is her name in Hello Nikki Let’s Beauty Up (Nikki Up2U 2 The World Travellers).
From the description of that hair I think Nikki was summoned to Miraland when she still looking for her missing father with her sister. Although both of official manga showed her being summoned from either in school and home.
Btw from first Nikki game, Nikki Up2U, her father is a photographer and one of the task is when her father have an exhibition though the person himself is never showed at all. In the second game, Nikki’s father gone missing and his daughters set to travel around the world to look for him. Here too both parents never shown. Not sure about now since English version no longer available, but it seems Chinese ver still available i-phone only.
There are few clothes tell about Nikki’s mother on current game. Apparently she is a beauty. Also from some characters reaction when they see Nikki and talk about Nikki being doing her predecessor’s dream or something (sorry will look on it again), probably hostess L is her mother (info from someone in FB spoiler group, its an old post)
Relatable FE Quote or Something 