Hmm... I suppose her mood would just drop and just say "I don't know". Like, it'll be very obvious that she wouldn't want to talk about her family, but if pressed for information she'll just ignore it. If she likes you she might elaborate, but only to say that she doesn't like to talk about her family (obviously). That being said if it's in a sort of threatening matter she might just take it as a joke, you know? Even if it's a serious threat, she'd blow it off with humor. So I suppose it depends on who is asking and in what context. 

If it's some stranger who comes up to her and asks her what she'd feel if her parents died, she might sarcastically say "before or after the party?" with a smile on her face, even though she'd be crying inside. If someone closer to her asks than they would get a more honest reaction from her. Nanuni is very good at being cheerful even when she really isn't, and while talking about family is one of the easier ways to crack her demeanor, you'd still have to really work at it.