Leave it to the Rainbow to lead. She may not refer to it at all, or be quite matter of fact about things. She may ask questions (she's likely to feel comfortable around you and ask stuff she may not feel able to ask her mum yet). She's not likely to want deep answers, just factual and age appropriate simple ones.
I've had to answer questions for my own daughter after her godmother was killed, I would emphasise that there's no right or wrong way to feel, that however she feels is the right way for her, even if it's different to how her mum or other family feels, and that its ok to laugh, be happy etc as well as cry and feel sad.
^^ that's a cut and paste from my response to a thread on the FB group from last year, the advice stands. Search for grief or similar on that group, there's a few threads that pop up which might be helpful.