She had already multiplied before Nux got there. Thats why she sued him and he found out he had a child. The others started to appear and owen was juggling 1 or 2 other Noi's. She started to feel lonely because of the loss of attention so she decided to find her other daddy and now we are here. The time span between nux finding out hes a dad to now is a 1 year and a half, although we might be up to 2 years now since they live together.
Its possible that more Noi's could pop up, I don't remember if owen said there was a cut off period were they stop appearing.
I mean I get that her dad is the same but how the heck did she multiple so suddenly? Maybe I missed the part when her dad did the same